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Global Warming: An Econometric Analysis

January 2013
SSRN Electronic Journal 2(4)

Authors:

University of Washington Seattle

An econometric analysis is provided of the system of relationships involving global CO2 emissions, atmospheric concentration of anthropogenic CO2, and global surface temperature increase since the preindustrial era. A key finding is that global surface temperature increase is estimated to have a positive effect on the CO2 absorption capacity of the environment.

Steady-state global surface temperature

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Journal of Finance and Economics

Volume 2, Issue 4 (2014), 17-24

ISSN 2291-4951 E-ISSN 2291-496X

Published by Science and Education Centre of North America

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Global Warming: An Econometric Analysis

Gary M Erickson1*

1Foster School of Business, University of Washington, Seattle, USA

*Correspondence: Gary M Erickson, Professor Emeritus, Foster School of Business, University of

Washington, Box 353200, Seattle, WA, 98195-3200, USA. Tel: 1-206-661-6112. E-mail:

erick@uw.edu

DOI: 10.12735/jfe.v2i4p17 URL: http://dx.doi.org/10.12735/jfe.v2i4p17

Abstract

An econometric analysis is provided of the system of relationships involving global CO2 emissions,

atmospheric concentration of anthropogenic CO2, and global surface temperature increase since the

preindustrial era. Empirical methods used are feasible generalized least squares and generalized

method of moments. It is found that global surface temperature increase is estimated to have a

positive effect on the CO2 absorption capacity of the environment.

JEL Classifications: C39, Q54

Keywords: anthropogenic CO2, atmospheric CO2 concentration, CO2 emissions, econometric

analysis, global warming

1. Introduction

There has been a considerable amount of science devoted to the study of the climate effects of

anthropogenic greenhouse gas (GHG) emissions and their increasing concentration in the

atmosphere. The Intergovernmental Panel on Climate Change (IPCC), in particular, has prepared its

Fifth Assessment Report (AR5) that includes three Working Group Reports and a Synthesis Report.

In addition, there continue to be numerous contributions to the academic literature on the subject.

Climate change is a well-studied phenomenon.

Even so, there remain unresolved issues regarding climate change and its attendant global

warming. An open matter of particular importance is whether increasing temperatures are affecting

the ability of the earth’s terrestrial and oceanic environments to absorb CO2, and if so, whether the

effect is positive or negative.

The present study aims to shed light on the global warming issue, in particular the effect of

warming on the earth’s ability to absorb CO2, through econometric analysis of available data on

fossil-fuel CO2 emissions, atmospheric concentration of CO2, and global temperature increase.

Econometric analysis can be insightful, in that such analysis is effective in the empirical study of

systems of relationships, and the interaction involving CO2 and global temperature is such a system.

The paper proceeds as follows. The next section summarizes the state of knowledge regarding

global warming, and is followed by sections that describe the data, outline the estimation model,

provide estimation results, and draw implications. A conclusions section finishes the paper.

Gary M. Erickson Submitted on September 18, 2014

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2. State of Knowledge

This section provides a summary of the essential basics of what is known, and not known, about

global warming related to anthropogenic GHG emissions. Stocker et al. (2013) provide a thorough

assessment of the current state of scientific knowledge.

There are two critical dynamic processes at work that are interrelated: (1) changes in

atmospheric concentrations of GHG; (2) changes in global surface temperature.

2.1 Changes in Atmospheric Concentrations of GHG

Concentrations of long-lived GHG, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O),

have increased substantially since the onset of the industrial revolution in the eighteenth century.

The increase is due to human activities, largely the burning of fossil fuels. The biggest contributor

to the radiative forcing of the climate system, and thereby global warming, is CO2, and so the

understanding of the influence of CO2 on global surface temperature is especially critical (Stocker

et al., 2013, Ch. 8).

The earth’s terrestrial and ocean environments are able to absorb CO2. After CO2 enters the

atmosphere, a third to half of the CO2 pulse is absorbed by land and ocean, with the remainder

removed through reaction with calcium carbonate over a few thousand years, and silicate

weathering over several hundred thousand years (Stocker et al., 2013, Ch. 6).

It is possible that the warming of the earth’s atmosphere is affecting the environment’s ability to

absorb CO2. However, this is not well understood. Nor is it well understood, if warming does affect

absorption capacity, whether the effect is positive or negative (Hansen, Kharecha, & Sato, 2013).

Studies offer apparently conflicting conclusions. Piao et al. (2008) and Zhao and Running (2010)

appear to find reduction in terrestrial carbon absorption in recent years, while Le Quéré et al. (2007)

and Schuster and Watson (2007) estimate decline in the ocean CO2 sink. On the other hand, Knorr

(2009) can find no trend, and Sarmiento et al. (2010) and Ballantyne, Alden, Miller, Tans, and

White (2012) observe increase in the environment’s net CO2 uptake since 1960. A contribution of

the present study is to shed an econometric light on this important issue.

2.2 Changes in Global Surface Temperature

The earth’s surface temperature is considered to adjust gradually to an increase in atmospheric CO2

toward an equilibrium. There are two key aspects of the relationship: the equilibrium temperature,

and the rate of adjustment toward the equilibrium.

Much effort has been dedicated, through a combination of historical temperature data analyses

and model simulations, to establish a range for “equilibrium climate sensitivity”, defined as the

global mean warming for a doubling of CO2 from the concentration level in the year 1750. The

IPCC’s Fifth Assessment Report (AR5) concludes that equilibrium climate sensitivity “…is likely in

the range 1.5oC to 4.5oC…” (Stocker et al., 2013, p. 1033). There remains uncertainty about the

precise ultimate change in temperature due to increasing atmospheric CO2.

The likely rate of adjustment to equilibrium can be gleaned from simulations done for IPCC’s

AR5. In particular, a scenario referred to as RCP4.5 shows stabilization in atmospheric CO2

concentration by about the year 2100, yet, for that scenario, mean surface air temperature is

projected to continue to rise by .5oC by 2200 and .7oC by 2300 (Stocker et al., 2013, Table 12.2).

The two data points in the temperature projections, coupled with an assumed exponential

adjustment relationship, suggest an annual adjustment rate of just about .01.

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3. Data

Data for the study are readily available from various sources.

Global fossil-fuel CO2 emissions data are provided, on an annual basis from 1751 through

2012, by U.S. Department of Energy’s Carbon Dioxide Information Analysis Center (CDIAC)

http://cdiac.ornl.gov/trends/emis/meth_reg.html. Emissions are measured in millions of metric tons

of carbon.

Annual atmospheric CO2 concentration measurements at Mauna Loa are available from Earth

System Research Laboratory of U.S. Department of Commerce’s National Oceanic & Atmospheric

Administration http://www.esrl.noaa.gov/gmd/ccgg/trends/, and are used with the permission of that

facility. CO2 concentration is measured as a dry mole fraction, number of molecules of CO2

divided by number of all molecules in air after water vapor has been removed, and expressed as

parts per million (ppm). Data are also available from 1980 on a global basis, but since the Mauna

Loa data closely match the global data, the Mauna Loa concentration data from 1959 are used in the

analysis. Further, since the interest is in anthropogenic CO2, we subtract the preindustrial

concentration level of 280 from the data.

Annual global surface temperatures are from National Aeronautics and Space Administration’s

Goddard Institute for Space Studies http://data.giss.nasa.gov/gistemp. Temperature is measured in

units of .01oC, and is expressed as the difference from the first year available, 1880, the temperature

for which year being used as a proxy for the preindustrial temperature level.

Finally, since annual variability in both temperature and CO2 concentration are correlated

with the Niño 3.4 index (Hansen, Ruedy, Sato, & Lo, 2010; Hansen et al., 2013), that index,

expressed as anomalies from the base period 1981-2010, is used as a covariate for both

relationships. The data on the Niño 3.4 anomalies are obtained from the National Weather

Service’s Climate Prediction Center http://www.cpc.ncep.noaa.gov/data/indices. Another index

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index.html is the AO

(Arctic Oscillation) index, which is related to temperatures in North America and Eurasia

(Thompson, Wallace, Jones, & Kennedy, 2009; Thompson, Wallace, & Hegerl, 2000), and so is

used as a covariate in the temperature relationship. Both indices are available on a monthly basis,

and are annualized by summing over the 12 months of the year.

Merging the various data provides an annual data base for the years 1959 through 2012.

4. Estimation Model

Define the following variables:

atmospheric concentration of anthropogenic CO2 in year t

global surface temperature increase over 1880 in year t

global CO2 emissions in year t

Niño 3.4 index level in year t

AO index level in year t.

Dependent variables for the econometric analysis are the annual changes in CO2 concentration and

temperature:

, .

t t t t tt

C C C T TT

−−

∆ = − ∆=−

Gary M. Erickson Submitted on September 18, 2014

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The global warming process is depicted as a system of two equations:

( )

1 11

1 1 11

t t t tt t

C M N TC

T N A CT

α β γδ µ

ε ζ ηθ ν

− −−

− − −−

∆= + −+ +

∆= + + − +

(1)

where

are parameters to be estimated, and

µν

are random disturbances

that are allowed to be correlated.

The first equation in the system (1) shows that the annual change in atmospheric CO2

concentration derives positively from CO2 emissions and negatively from environmental absorption

of atmospheric CO2. Annual change in the concentration is also affected by the Niño index. The

CO2 absorption rate depends on the temperature level, where the sign of the

parameter can be

positive or negative, and which sign will indicate whether temperature has a positive or negative

influence on the absorption rate. The model (1) allows the data to indicate the direction of the

influence.

The second equation in (1) shows that annual change in temperature is a partial adjustment to the

difference between the equilibrium temperature increase, which equilibrium depends on the current

CO2 concentration level, and the current temperature increase. Annual temperature change is also

influenced by the Niño and AO indices.

5. Estimation

Estimation of the nonlinear system (1) is accomplished using the Stata data analysis package. Two

estimation approaches are used. The first is maximum likelihood, specifically feasible generalized

least squares (FGLS), which allows the random disturbances for the two equations to be correlated.

The results from this estimation are shown in Table 1.

Table 1. FGLS estimation

Coefficient Estimate Standard error z

.0003863 .0001304 2.96

.0341525 .0070081 4.87

.0053833 .0115154 0.47

.0000875 .0000309 2.83

.4941749 .1909941 2.59

.3766008 .2272498 1.66

.6918035 .1176141 5.88

.7446362 .0275520 27.03

All coefficient estimates for the FGLS estimation are positive and statistically significant, except

for the base CO2 absorption rate, while the estimate for the AO effect on temperature change is

significant at only the .10 level. In particular, the estimate for the effect of temperature on the CO2

absorption rate is positive and significant. The point estimate values appear reasonable, except for

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that of the temperature adjustment parameter

, which is much higher than the .01 value suggested

by the RCP4.5 scenario.

The second estimation approach allows for correlation of regressors with the random

disturbances. This is likely the case, due to the interrelationship of atmospheric CO2 and

temperature exhibited in the system (1). The appropriate approach for this possibility is generalized

method of moments (GMM). Instruments are needed for this approach, and the following are used

as instruments in the estimation:

, , ,

tt t

MN A

−−

year, (year − 1959)^2. The estimation results are

shown in Table 2.

Table 2. GMM estimation

Coefficient Estimate Standard error z

.0002506 .0001166 2.15

.0378597 .0071695 5.28

−.0062871 .0102935 −0.61

.0000871 .0000245 3.55

.2136107 .2172791 0.98

.1606706 .2552154 0.63

.1027354 .2237836 0.46

.8803589 .3651523 2.41

The estimates for the CO2 concentration relationship are similar to those for FGLS estimation.

In particular, it is especially noteworthy that the estimated effect of temperature on the CO2

absorption rate is positive. Increase in global surface temperature appears to enhance the

environment’s ability to absorb CO2.

For the temperature relationship, the parameter estimates for GMM differ considerably from

those for FGLS. In particular, the estimate for the annual temperature adjustment parameter is much

smaller; in addition, the .01 value from the RCP4.5 scenario is within the 95% confidence interval

with GMM estimation, while it falls outside the interval with FGLS estimation.

6. Implications

Long-run implications for global temperatures can be drawn from the model (1) and its parameter

estimates. Consider the following continuous-time version of the model that disregards the short-

term Niño and AO effects and random disturbances:

( )

() () ()

() () .

dC Mt Tt Ct

dT Ct Tt

α γδ

ηθ

= −+

= −

(2)

For the system (2) to achieve a steady state, emissions M (t) must stabilize; assume such at a value

of M s, so that (2) becomes

Gary M. Erickson Submitted on September 18, 2014

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( )

() ()

() () .

dC M Tt Ct

dT Ct Tt

α γδ

ηθ

= −+

= −

(3)

Setting

0dC dt dT dt= =

achieves the following steady-state values for CO2 concentration and

temperature increase:

()

14.

γ γ αδθ

δθ

γ γ αδθ

∞

= −± +

(4)

It is straightforward to show that only the larger, positive, steady-state is feasible with nonnegative

initial values C(0), T(0):

()

14.

γ γ αδθ

δθ

γ γ αδθ

∞

= −+ +

(5)

Assume the GMM point estimates for the parameters

, except set

= 0, since that

parameter estimate is not statistically different from zero. This implies the following steady-state

relationships:

1.81

1.59

∞

(6)

and it can be shown that the steady state (6) is asymptotically stable. Since in the estimation

temperature is measured in terms of .01oC, the following converts the steady-state temperature

relationship in terms of oC:

.0159 .

∞

(7)

The relationship (7) is depicted in Figure 1. In particular, if global emissions could be stabilized

at the current level, around 10 billion metric tons, the global surface temperature would be expected

to rise to 1.59oC above the preindustrial level.

7. Conclusions

This paper provides an econometric analysis of the system of relationships involving global CO2

emissions, atmospheric concentration of anthropogenic CO2, and global surface temperature

increase.

A key finding is that, in both FGLS and GMM estimation of the system, temperature is

estimated to have a positive effect on the capacity of the earth’s environment to absorb CO2.

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Implications are drawn regarding the ultimate temperature consequences of differing levels of

stabilized CO2 emissions.

It is important to understand the nature of the interactive relationships that lead to global

warming. The econometric analysis herein provides a contribution to such understanding.

Figure 1. Steady-state global surface temperature

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Copyrights

granted to the journal. This is an open-access article distributed under the terms

and conditions of the Creative Commons Attribution 4.0 International License.

Optimal Control of Global Warming

Article

Dec 2012

Gary M. Erickson

An optimal control model of global warming is presented and analyzed. The model has two state variables, representing the anthropogenic concentration of greenhouse gases in the atmosphere and the temperature increase due to the greenhouse gas concentration. The state equation for greenhouse gas concentration includes absorption by the environment, but at a rate that is negatively related to temperature increase. Sufficient conditions are offered for a solution with two steady states to exist, and for at least one of the steady states to yield a positive absorption capacity rate. A numerical illustration is offered. Also, a special case of the model is analyzed, in which the environmental absorption capacity rate is assumed fixed.

Climate forcing growth rates: Doubling down on our Faustian bargain

Article

Full-text available

Mar 2013
ENVIRON RES LETT

Rahmstorf et al ’s (2012) conclusion that observed climate change is comparable to projections, and in some cases exceeds projections, allows further inferences if we can quantify changing climate forcings and compare those with projections. The largest climate forcing is caused by well-mixed long-lived greenhouse gases. Here we illustrate trends of these gases and their climate forcings, and we discuss implications. We focus on quantities that are accurately measured, and we include comparison with fixed scenarios, which helps reduce common misimpressions about how climate forcings are changing. Annual fossil fuel CO2 emissions have shot up in the past decade at about 3% yr⁻¹, double the rate of the prior three decades (figure 1). The growth rate falls above the range of the IPCC (2001) ‘Marker’ scenarios, although emissions are still within the entire range considered by the IPCC SRES (2000). The surge in emissions is due to increased coal use (blue curve in figure 1), which now accounts for more than 40% of fossil fuel CO2 emissions. Figure 1. CO2 annual emissions from fossil fuel use and cement manufacture, an update of figure 16 of Hansen (2003) using data of British Petroleum (BP 2012) concatenated with data of Boden et al (2012). The resulting annual increase of atmospheric CO2 (12-month running mean) has grown from less than 1 ppm yr⁻¹ in the early 1960s to an average ~2 ppm yr⁻¹ in the past decade (figure 2). Although CO2 measurements were not made at sufficient locations prior to the early 1980s to calculate the global mean change, the close match of global and Mauna Loa data for later years suggests that Mauna Loa data provide a good approximation of global change (figure 2), thus allowing a useful estimate of annual global change beginning with the initiation of Mauna Loa measurements in 1958 by Keeling et al (1973). Figure 2. Annual increase of CO2 based on data from the NOAA Earth System Research Laboratory (ESRL 2012). CO2 change and global temperature change are 12-month running means of differences for the same month of consecutive years. Nino index (Nino3.4 area) is 12-month running mean. Both temperature indices use data from Hansen et al (2010). Annual mean CO2 amount in 1958 was 315 ppm (Mauna Loa) and in 2012 was 394 ppm (Mauna Loa) and 393 ppm (Global). Interannual variability of CO2 growth is correlated with ENSO (El Nino Southern Oscillation) variations of tropical temperatures (figure 2). Ocean–atmosphere CO2 exchange is affected by ENSO (Chavez et al 1999), but ENSO seems to have a greater impact on atmospheric CO2 via the terrestrial carbon cycle through effects on the water cycle, temperature, and fire, as discussed in a large body of literature (referenced, e.g., by Schwalm et al 2011). In addition, volcanoes, such as the 1991 Mount Pinatubo eruption, slow the increase of atmospheric CO2 (Rothenberg et al 2012), at least in part because photosynthesis is enhanced by the increased proportion of diffuse sunlight (Gu et al 2003, Mercado et al 2009). Watson (1997) suggests that volcanic dust deposited on the ocean surface may also contribute to CO2 uptake by increasing ocean productivity. An important question is whether ocean and terrestrial carbon sinks will tend to saturate as human-made CO2 emissions continue. Piao et al (2008) and Zhao and Running (2010) suggest that there already may be a reduction of terrestrial carbon uptake, while Le Quéréet al (2007) and Schuster and Watson (2007) find evidence of decreased carbon uptake in the Southern Ocean and North Atlantic Ocean, respectively. However, others (Knorr 2009, Sarmiento et al 2010, Ballantyne et al 2012) either cast doubt on the reality of a reduced uptake strength or find evidence for increased uptake. An informative presentation of CO2 observations is the ratio of annual CO2 increase in the air divided by annual fossil fuel CO2 emissions (Keeling et al 1973), the ‘airborne fraction’ (figure 3, right scale). An alternative definition of airborne fraction includes in the denominator of this ratio an estimated net anthropogenic CO2 source from changes in land use, but this latter term is much more uncertain than the two terms involved in the Keeling et al (1973) definition. For example, analysis by Harris et al (2012) reveals a range as high as a factor of 2–4 in estimates of recent land use emissions; see also the discussion by Sarmiento et al (2010). However, note that the airborne fraction becomes smaller when estimated land use emissions are included, with the uptake fraction (one minus airborne fraction) typically greater than 0.5. Figure 3. Fossil fuel CO2 emissions (left scale) and airborne fraction, i.e., the ratio of observed atmospheric CO2 increase to fossil fuel CO2 emissions. Final three points are 5-, 3- and 1-year means. The simple Keeling airborne fraction, clearly, is not increasing (figure 3). Thus the net ocean plus terrestrial sink for carbon emissions has increased by a factor of 3–4 since 1958, accommodating the emissions increase by that factor. Remarkably, and we will argue importantly, the airborne fraction has declined since 2000 (figure 3) during a period without any large volcanic eruptions. The 7-year running mean of the airborne fraction had remained close to 60% up to 2000, except for the period affected by Pinatubo. The airborne fraction is affected by factors other than the efficiency of carbon sinks, most notably by changes in the rate of fossil fuel emissions (Gloor et al 2010). However, it is the dependence of the airborne fraction on fossil fuel emission rate that makes the post-2000 downturn of the airborne fraction particularly striking. The change of emission rate in 2000 from 1.5% yr⁻¹ to 3.1% yr⁻¹ (figure 1), other things being equal, would have caused a sharp increase of the airborne fraction (the simple reason being that a rapid source increase provides less time for carbon to be moved downward out of the ocean’s upper layers). A decrease in land use emissions during the past decade (Harris et al 2012) could contribute to the decreasing airborne fraction in figure 3, although Malhi (2010) presents evidence that tropical forest deforestation and regrowth are approximately in balance, within uncertainties. Land use change can be only a partial explanation for the decrease of the airborne fraction; something more than land use change seems to be occurring. We suggest that the huge post-2000 increase of uptake by the carbon sinks implied by figure 3 is related to the simultaneous sharp increase in coal use (figure 1). Increased coal use occurred primarily in China and India (Boden et al 2012; BP 2012; see graphs at www.columbia.edu/~mhs119/Emissions/Emis_moreFigs/). Satellite radiance measurements for July–December, months when desert dust does not dominate aerosol amount, yield an increase of aerosol optical depth in East Asia of about 4% yr⁻¹ during 2000–2006 (van Donkelaar et al 2008). Associated gaseous and particulate emissions increased rapidly after 2000 in China and India (Lu et al 2011, Tian et al 2010). Some decrease of the sulfur component of emissions occurred in China after 2006 as wide application of flue-gas desulfurization began to be initiated (Lu et al 2010), but this was largely offset by continuing emission increases from India (Lu et al 2011). We suggest that the surge of fossil fuel use, mainly coal, since 2000 is a basic cause of the large increase of carbon uptake by the combined terrestrial and ocean carbon sinks. One mechanism by which fossil fuel emissions increase carbon uptake is by fertilizing the biosphere via provision of nutrients essential for tissue building, especially nitrogen, which plays a critical role in controlling net primary productivity and is limited in many ecosystems (Gruber and Galloway 2008). Modeling (e.g., Thornton et al 2009) and field studies (Magnani et al 2007) confirm a major role of nitrogen deposition, working in concert with CO2 fertilization, in causing a large increase in net primary productivity of temperate and boreal forests. Sulfate aerosols from coal burning also might increase carbon uptake by increasing the proportion of diffuse insolation, as noted above for Pinatubo aerosols, even though the total solar radiation reaching the surface is reduced. Thus we see the decreased CO2 airborne fraction since 2000 as sharing some of the same causes as the decreased airborne fraction after the Pinatubo eruption (figure 3). CO2 fertilization is likely the major effect, as a plausible addition of 5 TgN yr⁻¹ from fossil fuels and net ecosystem productivity of 200 kgC kgN⁻¹ (Magnani et al 2007, 2008) yields an annual carbon drawdown of 1 GtC yr⁻¹, which is of the order of what is needed to explain the post-2000 anomaly in airborne CO2. However, an aerosol-induced increase of diffuse radiation might also contribute. Although tropospheric aerosol properties are not accurately monitored, there are suggestions of an upward trend of stratospheric background aerosols since 2000 (Hofmann et al 2009, Solomon et al 2011), which could be a consequence of more tropospheric aerosols at low latitudes where injection of tropospheric air into the stratosphere occurs (Holton et al 1995). We discuss climate implications of the reduced CO2 airborne fraction after presenting data for other greenhouse gases. Atmospheric CH4 is increasing more slowly than in IPCC scenarios (figure 4), which were defined more than a decade ago (IPCC 2001). However, after remaining nearly constant for several years, CH4 has increased during the past five years, pushing slightly above the level that was envisaged in the Alternative Scenario of Hansen et al (2000). Reduction of CH4, besides slowdown in CO2 growth in the twenty first century and a decline of CO2 in the twenty second century, is a principal requirement to achieve a low climate forcing that stabilizes climate, in part because CH4 also affects tropospheric ozone and stratospheric water vapor. The Alternative Scenario, defined in detail by Hansen and Sato (2004), keeps maximum global warming at ~1.5 °C relative to 1880–1920, under the assumption that fast-feedback climate sensitivity is ~3 °C for doubled CO2 (Hansen et al 2007). The Alternative Scenario allows CO2 to reach 475 ppm in 2100 before declining slowly; this scenario assumes that reductions of non-CO2 greenhouse gases and black carbon aerosols can be achieved sufficient to balance the warming effect of likely future decreases of reflective aerosols. Figure 4. Observed atmospheric CH4 amount and scenarios for twenty first century. Alternative scenario (Hansen et al 2000, Hansen and Sato 2004) yields maximum global warming ~1.5 °C above 1880–1920. Other scenarios are from IPCC (2001). Forcing on right hand scale is adjusted forcing, Fa, relative to values in 2000 (Hansen et al 2007). There are anthropogenic sources of CH4 that potentially could be reduced, indeed, the leveling off of CH4 amount during the past 20 years seems to have been caused by decreased venting in oil fields (Simpson et al 2012), but the feasibility of overall CH4 reduction also depends on limiting global warming itself, because of the potential for amplifying climate-CH4 feedbacks (Archer et al 2009, Koven et al 2011). Furthermore, reduction of atmospheric CH4 might become problematic if unconventional mining of gas, such as ‘hydro-fracking’, expands widely (Cipolla 2009), as discussed further below. The growth rate for the total climate forcing by well-mixed greenhouse gases has remained below the peak values reached in the 1970s and early 1980s, has been relatively stable for about 20 years, and is falling below IPCC (2001) scenarios (figure 5). However, the greenhouse gas forcing is growing faster than in the Alternative Scenario. MPTGs and OTGs in figure 5 are Montreal Protocol Trace Gases and Other Trace Gases (Hansen and Sato 2004). Figure 5. Five-year mean of the growth rate of climate forcing by well-mixed greenhouse gases, an update of figure 4 of Hansen and Sato (2004). Forcing calculations use equations of Hansen et al (2000). The moderate uncertainties in radiative calculations affect the scenarios and actual greenhouse gas results equally and thus do not alter the conclusion that the actual forcing falls below that of the IPCC scenarios. If greenhouse gases were the only climate forcing, we would be tempted to infer from Rahmstorf’s conclusion (that actual climate change has exceeded IPCC projections) and our conclusion (that actual greenhouse gas forcings are slightly smaller than IPCC scenarios) that actual climate sensitivity is on the high side of what has generally been assumed. Although that may be a valid inference, the evidence is weakened by the fact that other climate forcings are not negligible in comparison to the greenhouse gases and must be accounted for. Natural forcings, by changing solar irradiance and volcanic aerosols, are well-measured since the late 1970s and included in most IPCC (2007) climate simulations. The difficulty is human-made aerosols. Aerosols are readily detected in satellite observations, but determination of their climate forcing requires accurate knowledge of changes in aerosol amount, size distribution, absorption and vertical distribution on a global basis—as well as simultaneous data on changes in cloud properties to allow inference of the indirect aerosol forcing via induced cloud changes. Unfortunately, the first satellite mission capable of measuring the needed aerosol characteristics (Aerosol Polarimetry Sensor on the Glory satellite, (Mishchenko et al 2007)) suffered a launch failure and as yet there are no concrete plans for a replacement mission. The human-made aerosol climate forcing thus remains uncertain. IPCC (2007) concludes that aerosols are a negative (cooling) forcing, probably between -0.5 and -2.5 W m⁻². Hansen et al (2011), based mainly on analysis of Earth’s energy imbalance, derive an aerosol forcing -1.6 ± 0.3 W m⁻², consistent with an analysis of Murphy et al (2009) that suggests an aerosol forcing about -1.5 W m⁻² (see discussion in Hansen et al (2011)). This large negative aerosol forcing reduces the net climate forcing of the past century by about half (IPCC 2007; figure 1 of Hansen et al 2011). Coincidentally, this leaves net climate forcing comparable to the CO2 forcing alone. Reduction of the net human-made climate forcing by aerosols has been described as a ‘Faustian bargain’ (Hansen and Lacis 1990, Hansen 2009), because the aerosols constitute deleterious particulate air pollution. Reduction of the net climate forcing by half will continue only if we allow air pollution to build up to greater and greater amounts. More likely, humanity will demand and achieve a reduction of particulate air pollution, whereupon, because the CO2 from fossil fuel burning remains in the surface climate system for millennia, the ‘devil’s payment’ will be extracted from humanity via increased global warming. So is the new data we present here good news or bad news, and how does it alter the ‘Faustian bargain’? At first glance there seems to be some good news. First, if our interpretation of the data is correct, the surge of fossil fuel emissions, especially from coal burning, along with the increasing atmospheric CO2 level is ‘fertilizing’ the biosphere, and thus limiting the growth of atmospheric CO2. Also, despite the absence of accurate global aerosol measurements, it seems that the aerosol cooling effect is probably increasing based on evidence of aerosol increases in the Far East and increasing ‘background’ stratospheric aerosols. Both effects work to limit global warming and thus help explain why the rate of global warming seems to be less this decade than it has been during the prior quarter century. This data interpretation also helps explain why multiple warnings that some carbon sinks are ‘drying up’ and could even become carbon sources, e.g., boreal forests infested by pine bark beetles (Kurz et al 2008) and the Amazon rain forest suffering from drought (Lewis et al 2011), have not produced an obvious impact on atmospheric CO2. However, increased CO2 uptake does not necessarily mean that the biosphere is healthier or that the increased carbon uptake will continue indefinitely (Matson et al 2002, Galloway et al 2002, Heimann and Reichstein 2008, Gruber and Galloway 2008). Nor does it change the basic facts about the potential magnitude of the fossil fuel carbon source (figure 6) and the long lifetime of the CO2 in the surface carbon reservoirs (atmosphere, ocean, soil, biosphere) once the fossil fuels are burned (Archer 2005). Fertilization of the biosphere affects the distribution of the fossil fuel carbon among these reservoirs, at least on the short run, but it does not alter the fact that the fossil carbon will remain in these reservoirs for millennia. Figure 6. Fossil fuel CO2 emissions and carbon content (1 ppm atmospheric CO2~2.12 GtC). Historical emissions are from Boden et al (2012). Estimated reserves and potentially recoverable resources are based on energy content values of Energy Information Administration (EIA 2011), German Advisory Council (GAC 2011), and Global Energy Assessment (GEA 2012). We convert energy content to carbon content using emission factors of Table 4.2 of IPCC (2007) for coal, gas, and conventional oil, and, following IPCC, we use an emission factor of unconventional oil the same as that for coal. Humanity, so far, has burned only a small portion (purple area in figure 6) of total fossil fuel reserves and resources. Yet deleterious effects of warming are apparent (IPCC 2007), even though only about half of the warming due to gases now in the air has appeared, the remainder still ‘in the pipeline’ due to the inertia of the climate system (Hansen et al 2011). Already it seems difficult to avoid passing the ‘guardrail’ of no more than 2 °C global warming that was agreed in the Copenhagen Accord of the United Nations Framework Convention on Climate Change (UNFCCC 2010). And Hansen et al (2008), based primarily on paleoclimate data and evidence of deleterious climate impacts already at 385 ppm CO2, concluded that an appropriate initial target for CO2 was 350 ppm, which implied a global temperature limit, relative to 1880–1920 of about 1 °C. What is clear is that most of the remaining fossil fuels must be left in the ground if we are to avoid dangerous human-made interference with climate. The principal implication of our present analysis probably relates to the Faustian bargain. Increased short-term masking of greenhouse gas warming by fossil fuel particulate and nitrogen pollution represents a ‘doubling down’ of the Faustian bargain, an increase in the stakes. The more we allow the Faustian debt to build, the more unmanageable the eventual consequences will be. Yet globally there are plans to build more than 1000 coal-fired power plants (Yang and Cui 2012) and plans to develop some of the dirtiest oil sources on the planet (EIA 2011). These plans should be vigorously resisted. We are already in a deep hole—it is time to stop digging. Acknowledgments We thank ClimateWorks, Energy Foundation, Gerry Lenfest (Lenfest Foundation), Lee Wasserman (Rockefeller Family Foundation), and Stephen Toben (Flora Family Foundation) for research and communications support. References Archer D 2005 Fate of fossil fuel CO2 in geologic time J. Geophys. Res. 110 C09505 Archer D, Buffett B and Brovkin V 2009 Ocean methane hydrates as a slow tipping point in the global carbon cycle Proc. Natl Acad. 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Identifying Signatures of Natural Climate Variability in Time Series of Global-Mean Surface Temperature: Methodology and Insights

Article

Full-text available

Nov 2009

Global-mean surface temperature is affected by both natural variability and anthropogenic forcing. This study is concerned with identifying and removing from global-mean temperatures the signatures of natural climate variability over the period January 1900-March 2009. A series of simple, physically based methodologies are developed and applied to isolate the climate impacts of three known sources of natural variability: the El Nino-Southern Oscillation (ENSO), variations in the advection of marine air masses over the high-latitude continents during winter, and aerosols injected into the stratosphere by explosive volcanic eruptions. After the effects of ENSO and high-latitude temperature advection are removed from the global-mean temperature record, the signatures of volcanic eruptions and changes in instrumentation become more clearly apparent. After the volcanic eruptions are subsequently filtered from the record, the residual time series reveals a nearly monotonic global warming pattern since similar to 1950. The results also reveal coupling between the land and ocean areas on the interannual time scale that transcends the effects of ENSO and volcanic eruptions. Globally averaged land and ocean temperatures are most strongly correlated when ocean leads land by; 2-3 months. These coupled fluctuations exhibit a complicated spatial signature with largest-amplitude sea surface temperature perturbations over the Atlantic Ocean.

Annular Modes in the Extratropical Circulation. Part II: Trends

Article

Full-text available

Mar 2000

The authors exploit the remarkable similarity between recent climate trends and the structure of the `annular modes' in the month-to-month variability (as described in a companion paper) to partition the trends into components linearly congruent with and linearly independent of the annular modes.The index of the Northern Hemisphere (NH) annular mode, referred to as the Arctic Oscillation (AO), has exhibited a trend toward the high index polarity over the past few decades. The largest and most significant trends are observed during the `active season' for stratospheric planetary wave-mean flow interaction, January-March (JFM), when fluctuations in the AO amplify with height into the lower stratosphere. For the periods of record considered, virtually all of the JFM geopotential height falls over the polar cap region and the strengthening of the subpolar westerlies from the surface to the lower stratosphere, 50% of the JFM warming over the Eurasian continent, 30% of the JFM warming over the NH as a whole, 40% of the JFM stratospheric cooling over the polar cap region, and 40% of the March total column ozone losses poleward of 40°N are linearly congruent with month-to-month variations in the AO index. Summertime sea level pressure falls over the Arctic basin are suggestive of a year-round drift toward the positive polarity of the AO, but the evidence is less conclusive. Owing to the photochemical memory inherent in the ozone distribution, roughly half the ozone depletion during the NH summer months is linearly dependent on AO-related ozone losses incurred during the previous active season.Lower-tropospheric geopotential height falls over the Antarctic polar cap region are indicative of a drift toward the high index polarity of the Southern Hemisphere (SH) annular mode with no apparent seasonality. In contrast, the trend toward a cooling and strengthening of the SH stratospheric polar vortex peaks sharply during the stratosphere's relatively short active season centered in November. The most pronounced SH ozone losses have occurred in September-October, one or two months prior to this active season. In both hemispheres, positive feedbacks involving ozone destruction, cooling, and a weakening of the wave-driven meridional circulation may be contributing to a delayed breakdown of the polar vortex and enhanced ozone losses during spring.

Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years

Article

Full-text available

Aug 2012
NATURE

One of the greatest sources of uncertainty for future climate predictions is the response of the global carbon cycle to climate change. Although approximately one-half of total CO(2) emissions is at present taken up by combined land and ocean carbon reservoirs, models predict a decline in future carbon uptake by these reservoirs, resulting in a positive carbon-climate feedback. Several recent studies suggest that rates of carbon uptake by the land and ocean have remained constant or declined in recent decades. Other work, however, has called into question the reported decline. Here we use global-scale atmospheric CO(2) measurements, CO(2) emission inventories and their full range of uncertainties to calculate changes in global CO(2) sources and sinks during the past 50 years. Our mass balance analysis shows that net global carbon uptake has increased significantly by about 0.05 billion tonnes of carbon per year and that global carbon uptake doubled, from 2.4 ± 0.8 to 5.0 ± 0.9 billion tonnes per year, between 1960 and 2010. Therefore, it is very unlikely that both land and ocean carbon sinks have decreased on a global scale. Since 1959, approximately 350 billion tonnes of carbon have been emitted by humans to the atmosphere, of which about 55 per cent has moved into the land and oceans. Thus, identifying the mechanisms and locations responsible for increasing global carbon uptake remains a critical challenge in constraining the modern global carbon budget and predicting future carbon-climate interactions.

2 A variable and decreasing sink for atmospheric CO2 in the North 3 Atlantic

Article

Full-text available

Nov 2007
J GEOPHYS RES

A time series of observations from merchant ships between the U.K. and the Caribbean is used to establish the variability of sea surface pCO2 and air-to-sea flux from the mid-1990s to early 2000s. We show that the sink for atmospheric CO2 exhibits important interannual variability, which is in phase across large regions from year to year. Additionally, there has been an interdecadal decline, evident throughout the study region but especially significant in the northeast of the area covered, with the sink reducing >50% from the mid-1990s to the period 2002-2005. A review of available observations suggests a large region of decrease covering much of the North Atlantic but excluding the western subtropical areas. We estimate that the uptake of the region between 20°N and 65°N declined by ∼0.24 Pg C a-1 from 1994/1995 to 2002-2005. Declining rates of wintertime mixing and ventilation between surface and subsurface waters due to increasing stratification, linked to variation in the North Atlantic Oscillation, are suggested as the main cause of the change. These are exacerbated by a contribution from the changing buffer capacity of the ocean water, as the carbon content of surface waters increases.

Global Surface Temperature Change

Article

Full-text available

Dec 2010

We update the Goddard Institute for Space Studies (GISS) analysis of global surface temperature change, compare alternative analyses, and address questions about perception and reality of global warming. Satellite-observed nightlights are used to identify measurement stations located in extreme darkness and adjust temperature trends of urban and peri-urban stations for non-climatic factors, verifying that urban effects on analyzed global change are small. Because the GISS analysis combines available sea surface temperature records with meteorological station measurements, we test alternative choices for the ocean data, showing that global temperature change is sensitive to estimated temperature change in polar regions where observations are limited. We use simple 12-month (and n×12) running means to improve the information content in our temperature graphs. Contrary to a popular misconception, the rate of warming has not declined. Global temperature is rising as fast in the past decade as in the prior two decades, despite year-to-year fluctuations associated with the El Nino-La Nina cycle of tropical ocean temperature. Record high global 12-month running-mean temperature for the period with instrumental data was reached in 2010.

Trends and regional distributions of land and ocean carbon sinks

Article

Full-text available

Aug 2010

We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 Pg C yr−1 between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) Pg C yr−1 to a new relatively constant mean of −1.15 Pg C yr−1 between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO2, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

Climate Change 2013: The Physical Science Basis

Technical Report

Jan 2013

Is the airborne fraction of anthropogenic CO2 emissions increasing?

Article

Nov 2009

Wolfgang Knorr

Several recent studies have highlighted the possibility that the oceans and terrestrial ecosystems have started losing part of their ability to sequester a large proportion of the anthropogenic CO2 emissions. This is an important claim, because so far only about 40% of those emissions have stayed in the atmosphere, which has prevented additional climate change. This study re-examines the available atmospheric CO2 and emissions data including their uncertainties. It is shown that with those uncertainties, the trend in the airborne fraction since 1850 has been 0.7 ± 1.4% per decade, i.e. close to and not significantly different from zero. The analysis further shows that the statistical model of a constant airborne fraction agrees best with the available data if emissions from land use change are scaled down to 82% or less of their original estimates. Despite the predictions of coupled climate-carbon cycle models, no trend in the airborne fraction can be found.

Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009

Article

Aug 2010
SCIENCE

Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.

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