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Greenhouses, hot water bottles, cycles and the future of New Zealand climate

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Abstract

Greenhouse effect The Greenhouse Effect involves the interaction of infrared or long-wave radiation (0.8-1000 µm wavelengths) emitted by the earth, with most types of molecules in the atmosphere. The Greenhouse Effect is most effective between 3 and 20 µm (longer wavelengths are associated with colder temperatures, with 14 µm corresponding to snow and ice), although there are some wavelengths in this range where there is almost no interaction. Infrared radiation is absorbed by the molecules, and eventually re-emitted at longer wavelengths, resulting in a reduction in the rate at which outgoing longwave radiation (OLR) escapes from the atmosphere. Assuming that all other components of the earth's climate system remain constant, and given an infinite flat earth and well-mixed atmosphere, an increase in greenhouse gas concentration leads to an increase in the equilibrium temperature of the earth. Assuming a spherical finite earth, the result is less clear-cut, and suggests that increasing concentrations may have little effect. However, both approaches indicate that the direct increase in temperature due to an enhanced Greenhouse Effect is relatively small, and positive feedback mechanisms are invoked to predict higher temperature responses. Ocean thermal energy Concurrently with increasing global air surface temperature, there has been an increase in global sea surface temperatures, and the total thermal energy in the upper 700 m of the oceans (often referred to as Ocean Heat). Some studies have shown that the increases and decreases in ocean heat precede the rise and fall of atmospheric surface temperatures over continents, and major weather fluctuations such as the El-Niño Southern Oscillation (ENSO) also follow changes in the quantity and distribution of ocean heat. The oceans derive 99.9% of their thermal energy from solar short-wave radiation (0.4-1 µm), with most of the remaining 0.1% due to the geothermal heat flux from within the earth. A small amount of thermal energy is produced by the dissipation of waves, particularly long-period tidal waves. The amount of energy absorbed at infrared wavelengths (>1 µm) is negligible, implying Abstract The Greenhouse Effect acts to slow the escape of infrared radiation to space, and hence warms the atmosphere. The oceans derive almost all of their thermal energy from the sun, and none from infrared radiation in the atmosphere. The thermal energy stored by the oceans is transported globally and released after a range of different time periods. The release of thermal energy from the oceans modifies the behaviour of atmospheric circulation, and hence varies climate. Based on ocean behaviour, New Zealand can expect weather patterns similar to those from 1890-1922 and another Little Ice Age may develop this century.
205
Greenhouses, hot water bottles, cycles and the future of
New Zealand climate
W.P. DE LANGE
Department of Earth & Ocean Sciences, University of Waikato
delange@waikato.ac.nz
Greenhouse effect
The Greenhouse Effect involves the interaction
of infrared or long-wave radiation (0.8-1000 µm
wavelengths) emitted by the earth, with most types
of molecules in the atmosphere. The Greenhouse
Effect is most effective between 3 and 20 µm (longer
wavelengths are associated with colder temperatures,
with 14 µm corresponding to snow and ice), although
there are some wavelengths in this range where there
is almost no interaction. Infrared radiation is absorbed
by the molecules, and eventually re-emitted at longer
wavelengths, resulting in a reduction in the rate at which
outgoing longwave radiation (OLR) escapes from the
atmosphere. Assuming that all other components of
the earth’s climate system remain constant, and given
an innite at earth and well-mixed atmosphere, an
increase in greenhouse gas concentration leads to an
increase in the equilibrium temperature of the earth.
Assuming a spherical nite earth, the result is less
clear-cut, and suggests that increasing concentrations
may have little effect. However, both approaches
indicate that the direct increase in temperature due to
an enhanced Greenhouse Effect is relatively small, and
positive feedback mechanisms are invoked to predict
higher temperature responses.
Ocean thermal energy
Concurrently with increasing global air surface
temperature, there has been an increase in global sea
surface temperatures, and the total thermal energy in the
upper 700 m of the oceans (often referred to as Ocean
Heat). Some studies have shown that the increases
and decreases in ocean heat precede the rise and fall
of atmospheric surface temperatures over continents,
and major weather uctuations such as the El-Niño
Southern Oscillation (ENSO) also follow changes in
the quantity and distribution of ocean heat.
The oceans derive 99.9% of their thermal energy from
solar short-wave radiation (0.4-1 µm), with most of the
remaining 0.1% due to the geothermal heat ux from
within the earth. A small amount of thermal energy is
produced by the dissipation of waves, particularly long-
period tidal waves. The amount of energy absorbed at
infrared wavelengths (>1 µm) is negligible, implying
Abstract
The Greenhouse Effect acts to slow the escape of infrared
radiation to space, and hence warms the atmosphere.
The oceans derive almost all of their thermal energy
from the sun, and none from infrared radiation in the
atmosphere. The thermal energy stored by the oceans
is transported globally and released after a range of
different time periods. The release of thermal energy
from the oceans modies the behaviour of atmospheric
circulation, and hence varies climate. Based on ocean
behaviour, New Zealand can expect weather patterns
similar to those from 1890-1922 and another Little Ice
Age may develop this century.
Introduction
The moisture and varying temperature of the land depends
largely upon the positions of the currents in the ocean,
and it is thought that when we know the laws of the latter
we will, with the aid of meteorology, be able to say to the
farmers hundreds of miles distant from the sea, “you will
have an abnormal amount of rain during next summer,”
or, “the winter will be cold and clear,” and by these
predictions they can plant a crop to suit the circumstances
or provide an unusual amount of food for their stock (Lt.
John E. Pillsbury from The Gulf Stream, Appendix 10, US
Coast & Geodetic Survey report, 1891).
Geological evidence indicates that the earth has had
oceans and an atmosphere for the last 3.8 billion years,
and during this time climate has always been varying.
The most recent assessment of the Intergovernmental
Panel on Climate Change (IPCC) was that since AD
1950, Anthropogenic Global Warming (AGW), which
is now referred to as Climate Change in the media,
has superseded natural climate change. The dominant
factor is inferred to be an increase in the radiative
forcing associated with the Greenhouse Effect, due
to an increased concentration of some greenhouse
gases resulting from human activities. However, there
are other anthropogenic factors that may inuence
climate, such as land use changes and the discharge of
particulates into the atmosphere. Further, the analysis
is based on the assumption that only radiative forcing
involving outgoing infrared radiation has a signicant
effect on global climate.
206 (2009)Proceedings of the New Zealand Grassland Association 71: 000–000
that the Greenhouse Effect does not heat the oceans to
any signicant degree.
Hot water bottle effect
Water has a high heat capacity, which gives it the ability
to absorb, store and emit thermal energy; hence, the use
of hot water bottles to provide warmth. The top 3.2 m of
the ocean has the same total heat capacity as the entire
atmosphere, which means that the oceans store most of
the energy in the global climate system. The hot water
bottle effect describes the movement of heat. The ocean
warms up if it absorbs heat faster than it loses heat,
and it cools down if it loses heat faster than absorbing
it. This is clearly evident in the annual cycle of both
surface water temperatures and sea level. At the tropics
and higher latitudes, maximum surface temperatures
and the associated maximum thermal expansion occur
in late summer to early autumn, with the corresponding
minima in late winter to early spring.
At longer time scales, variations in the rate of solar
warming are associated with: uctuations in solar output
in terms of both the amount of energy available, and the
distribution of energy across the different wavelengths;
and the amount of solar energy transmitted to the oceans,
which uctuates with orbital distance, the orientation
of earth, and changes in atmospheric absorbance
and albedo (mostly the solar energy absorbed and
reected by clouds). Although variations in Total Solar
Irradiance (TSI or visible light) appear to be small,
there is growing evidence that these small changes
are amplied by both: pressure uctuations in the
stratosphere linked to absorption of UV by ozone; and
variations in evaporation rates affecting cloud clover,
and hence albedo, in cloud-free subtropical areas.
Further, variations in solar activity are not conned to
TSI; much larger variations have been observed in the
strength of the sun’s magnetic eld, and it’s likely they
cause uctuations in the earth’s albedo through changes
in cloud cover.
General Circulation Models (GCMs) used to project
future climate have signicantly different equilibrium
climate sensitivities, ranging from 2 to 5°C for a
doubling of effective carbon dioxide. Much of this
difference is attributed to different treatment of clouds
and associated feedbacks, particularly those associated
with deep convective clouds (the tallest clouds), and
thin marine layer (low level) clouds. GCMs treat deep
convective clouds as a positive feedback (warming),
while thin marine layer clouds are ignored or treated as a
weak negative feedback. Initial satellite data (Nimbus-7
1980-84) suggested that, on average, clouds cover 51.8%
of the earth. However, with the steady improvement in
sensor resolution and spectral sensitivity, the coverage
has been revised to 62.7% (ISCCP C2 1984-88), and
more recently 77.6% (MODIS 2000-06). Most of the
extra coverage represents thin marine layer clouds,
suggesting GCMs underestimate the negative feedback
associated with these clouds.
Although the data presented above may suggest
an increase in global cloud cover, analysis of the
individual datasets indicates that overall cloud coverage
has declined from 1980-2006, contrary to the positive
feedback assumed in GCMs. It has been hypothesised
that the reduction was due to a declining availability
of ionised nuclei required for cloud formation,
since the sun’s stronger magnetic eld reduced the
concentration of galactic cosmic rays (GCRs) entering
the atmosphere. In particular, the GCR ux is thought to
affect the distribution of thin marine layer clouds, and
hence the shortwave radiation uxes into the oceans.
However, since the solar maximum during Solar Cycle
23 (2000-2002), there has been a dramatic drop in both
the sunspot and overall solar magnetic eld, coinciding
with extended periods of no sunspot activity, which
may be contributing to increased marine layer cloud
formation, leading to cooling. In addition to there being
very few sunspots, the strength of the sunspots that do
occur is much less than 20 years ago.
Ocean currents redistribute the heat entering the
ocean from the sun. There is a higher solar ux near
the equator than at the poles, so both the oceans and
atmosphere are warmer near the equator. Atmospheric
and oceanic circulation both transport energy from
the equator to the poles as a result of the thermal
gradient. However, due to the presence of land masses
that constrain ocean circulation, the transport in the
oceans is not uniform. Presently, the oceans effectively
transport thermal energy from the North Pacic Ocean
to the North Atlantic, resulting in a higher evaporation
rate in the North Atlantic, which is balanced by a higher
precipitation rate in the North Pacic. This results in
the water in the North Atlantic being warmer and saltier
than the water in the North Pacic. Cold fresher water
can freeze more easily than warm saltier water, so the
formation of sea ice in the arctic is controlled by the
relative inows of Pacic and Atlantic water into the
Arctic Ocean. There are similar, but less pronounced
salinity/temperature anomalies around Antarctica, that
rotate around the continent (Antarctic Circumpolar
Wave) and inuence sea ice formation.
There are also processes that affect the rate at which
oceans lose thermal energy. On average oceans lose
53% as latent heat due to evaporation, 41% as infrared
emissions, and 6% by conduction to the atmosphere.
However, these losses can vary in response to the
amount of turbulence and extent of cloud cover, which
provide a feedback mechanism to limit the total amount
of ocean heat in the upper ocean. Over long time
207
periods, the location of the pycnocline (the boundary
between warmer surface waters and colder deep
waters) is quite stable, indicating that the total amount
of thermal energy stored in the surface and deep oceans
is relatively constant. At shorter time periods, there do
appear to be uctuations, particularly in the surface
oceans, that are associated with climate variability.
Ocean-atmosphere interactions and
climate variability
The Pacic warm pool
The international TOGA COARE project conducted
in 1992-93 measured ocean-atmosphere interactions
involving the “warm pool” region of the western Pacic.
This area contains some of the warmest ocean surface
waters on earth and plays an important role in global
climate variability, particularly ENSO. COARE found
that several processes limited the maximum surface
temperature of the pool, and also the total volume of
warm water.
As the surface temperature increased in response
to solar forcing, there was an increase in evaporation
that ultimately resulted in an increase in precipitation,
transferring energy from the ocean to the atmosphere.
The precipitation also predominantly fell back into
the ocean, creating a cold fresh pool on the ocean
surface that cooled the warm pool as it mixed with the
underlying warmer saltier water. More recent studies
have shown that as the ocean surface temperature
increases, the strength of convection in the overlying
atmosphere also increases. This results in taller but
narrower clouds, so although the thickness of the cloud
layer increases, the area covered decreases, allowing
more OLR to leave the atmosphere, hence cooling the
ocean. The Earth Radiation Budget Experiment (ERBE)
has observed that within the tropics, an increase in sea
surface temperature does increase OLR, contrary to the
assumed response in GCMs.
The warm pool occasionally ows eastward along
the equator, resulting in anomalously warm waters
in the eastern Pacic Ocean, particularly off the
Southern American coast. This is associated with
changes in the strength and direction of atmospheric
circulation systems, collectively known as an El Niño
event within the ENSO cycle. The redistribution of
the waters of the warm pool results in an increase in
OLR and consequently an increase in the global air
surface temperature after a lag of about 7 months. The
magnitude of the El Niño event is related to the volume
of the warm water pool and the proportion that ows
eastward (the largest volume measured to date preceded
the 1998 super-El Niño). Following an El Niño event
warm anomalies have been observed propagating around
the oceans, eventually arriving in the North Atlantic.
The discredited “Hockey Stick” reconstruction of
global temperature over the past millennium disputed
the existence of a global temperature maximum known
as the Mediaeval Warm Period. It was argued that the
multiplicity of proxy data showing such a warm period
were due to regional effects, not global warming.
The Pacic Warm Pool has undergone temperature
uctuations over the last millennium that supports
both a Mediaeval Warm Period and a Little Ice Age,
followed by a Modern Warm Period that is no warmer
than the Mediaeval Warm Period. Given the global
impact that the Warm Pool has on climate and the
increasingly global proxy evidence, it is clear that the
Mediaeval Warm Period was global, and the “Hockey
Stick” interpretation is incorrect.
Ocean circulation and climate oscillations
Water density is an important factor controlling ocean
circulation, and is a function of temperature, salinity
and pressure. Water masses with different temperatures
and salinities can have the same density and therefore
occur at the same depth in the ocean. Below the
surface, the temperature and salinity of water masses
change slowly, mostly in response to mixing. Further,
water masses move more easily horizontally, since
vertical movement requires a change in density. At the
surface, interactions with the atmosphere can result in
rapid changes in temperature and salinity, which can
result in density changes, or the temperature changes
are compensated for by salinity changes resulting in
no density change (known as changes to the spiciness
of the water). Typically, warming at the surface is
associated with an increase in salinity, while cooling is
associated with a decrease in salinity. This phenomenon
results in warmer, saltier (spicy) water masses with
the same density as cooler, fresher (non-spicy) water
masses, which all ow at the same depth and provide
a “memory” of the ocean/atmosphere interactions that
created them.
In the Pacic Ocean, there are at least four layers
in which spiciness anomalies can move and affect
weather and climate. At the very surface, anomalies
propagate slowly eastward in association with a 30-
60 day weather cycle known as the Madden-Julian
Oscillation (MJO). The MJO involves periods of
both increased convection that can spawn tropical
cyclones and greatly reduced convection that can create
drought conditions. At slightly greater depths, there is
an eastward ow associated with ENSO. This ow is
deeper in the central Pacic and surfaces in the eastern
Pacic (re-emergence), where it produces anomalously
high sea surface temperatures and an increase in OLR
as discussed earlier.
The deeper layers are less well understood and involve
Greenhouses, hot water bottles, cycles and the future of New Zealand climate (W.P. de Lange)
208 (2009)Proceedings of the New Zealand Grassland Association 71: 000–000
more complicated ow paths. Immediately above the
major density boundary in the ocean (the pycnocline)
are anomalies linked to the Pacic Decadal Oscillation
(PDO) and Atlantic Multidecadal Oscillation (AMO).
These phenomena have periods of 60-80 years, which
is consistent with the movement of water around the
oceans along the pycnocline. The pycnocline is bowl-
shaped, so it is deepest in the mid-ocean basins of
the North and South Pacic and Atlantic oceans, and
nears the surface at high latitudes and the Equator.
Anomalous water masses created at high latitudes,
particularly the North Pacic and North Atlantic, ow
horizontally along the pycnocline, and rotate around
the ocean basins, eventually re-emerging at high
latitudes. Depending on weather conditions when the
water masses re-emerge, the spiciness anomalies can be
strengthened or diluted.
In the Pacic Ocean it is also evident that due to the
shallow pycnocline, anomalous spiciness associated
with the PDO can interact with the overlying water
masses associated with ENSO. If a spicy PDO anomaly
coincides with the warm pool, it can signicantly
increase the volume and warmth of the pool and
strengthen the subsequent El Niño event. Similarly, a
non-spicy PDO anomaly will weaken El Niño events
and favour neutral or La Niña conditions. Proxy and
historical data indicate that this pattern has consistently
occurred for the last 400 years, and most probably
occurred for the last several millennia.
Spiciness also occurs in Intermediate Waters located
just below the pycnocline in the oceans. These waters
originate at higher latitudes than the anomalies involved
in the PDO, particularly around Antarctica where they
are affected by the extent of sea ice and the strength of
circum-Antarctic circulation. At this stage it is unclear
what role the anomalies play in global climate, but they
may be associated with the 1470 year Bond Cycles
evident in the palaeoclimate record. Bond Cycles are
most pronounced during glacial times when they are
linked to the inux of coarse sediment into the deep
ocean, particularly in the North Atlantic. The pattern
of Warm Periods and Little Ice Ages over the last few
millennia has been linked to Bond Cycles, although the
mechanisms involved are not known.
Although the Intermediate Water spiciness anomalies
lie beneath the pycnocline, which restricts vertical
mixing, these waters can re-emerge at high latitudes, and
near the equator they can interact with surface waters
due to the shallowness of the pycnocline. This suggests
that these anomalies can interact with the Pacic Warm
Pool in a similar fashion to those associated with the
PDO. Analysis of millennial scale proxy data for the
Pacic Warm Pool provides evidence that since the
Last Glacial Maximum there have been uctuations in
extent and maximum temperature of the pool consistent
with Bond Cycles. This may provide a mechanism to
drive warm/cool climate variability at millennial time
scales.
New Zealand climate variability
New Zealand climate, particularly rainfall and wind
patterns, shows systematic variations at different time
periods. Due to the short instrumental record, most
of the identied variation is strictly weather and not
climate, and includes: a quasi-biennial oscillation
(QBO) associated with sea-level pressure and
meridional (north-south) ow around New Zealand;
the ENSO pattern with periods of 3-8 years; a decadal
pattern strongly correlated with the 11 year Schwabe
sunspot cycle; and cycles with periods of 18-22 years
that also correlate well with the Hale magnetic solar
cycle. A 70-80 year pattern linked to the PDO is also
evident, which some have correlated to the 60-120 year
Gleissberg cycle that is associated with modulation of
the Schwabe cycle amplitude. Proxy data also suggest
the presence of a 200-220 year de Vries solar cycle
(also known as Seuss Cycle).
It is recognised that both the QBO and ENSO
undergo sudden phase shifts that affect their amplitude
and inuence on New Zealand, as do the strong circum-
Antarctic (zonal) westerly winds (ZWW) to the south
of New Zealand. Some of these phase shifts correlate
well with the Schwabe solar cycle, and others with
the phase shifts of the PDO. In particular, three major
climate shifts have been identied for New Zealand
from instrumental data: 1922-1944 positive PDO; 1946-
1977 negative PDO; and 1978-1998 positive PDO. Since
2002, there has been a shift to a negative PDO.
There are three regions of coherent sea surface
temperature around New Zealand: the Tasman sea to the
west; and two areas towards the east, north and south of
the Chatham Rise (which constrains the location of the
Sub-Tropical Front (STF) separating waters sourced
from high and low latitudes). ENSO events affect the
sea surface temperatures in the Tasman Sea (cooler
during El Niño), and the PDO affects sea surface
temperatures east of New Zealand around the STF. In
turn these affect the temperature and pressure gradients
across New Zealand.
A positive PDO coincides with an increase in mean
sea level pressure (SLP) over the Tasman sea (west
of 170°W), a decrease in sea surface temperature east
of New Zealand, and the South Pacic Convergence
Zone (SPCZ) moving northeast. This results in an
increased west to southwest wind ow across New
Zealand, a reduction in precipitation in the north of
the North Island, and an increased precipitation for the
north, west, south and southeast of the South Island.
209
Storm surges are smaller and less frequent, and there
is a reduction in the frequency of waterspouts (coastal
tornadoes) and mesoscale storms. The opposite occurs
during a negative PDO.
The transition from one state to the other of the PDO
coincides with a step-like increase/decrease in sea level,
which appears to match a similar increase/decrease
in global surface air temperature and upper ocean
temperature, consistent with thermal expansion being
the main driver of sea-level change. Globally, a positive
PDO is associated with higher temperatures, and a
negative PDO with cooler conditions. New Zealand’s
temperature response is more complicated, as increased
southwest wind ows tend to lower temperatures.
Future New Zealand climate
The IPCC developed a range of scenarios to estimate
future concentrations of Greenhouse Gases, and these
were used as the basis of projections of future climate.
They are not forecasts in the same sense as weather
forecasts, and therefore it is difcult to assess the
signicance of any particular projection. It is clear
that all the scenarios used have overestimated the
concentrations of the Greenhouse Gases considered
for the rst decade of this century, even though it is
argued that the estimated emissions have exceeded the
assumed emissions for some scenarios.
Based on the projections, the IPCC considered the
most likely rate of increase in global temperature this
century to be 0.2° per decade, and the rate of sea level
rise to be 4 cm per decade. The other projections of
climate change vary widely between models and there
is no consistent projection that can be applied to New
Zealand. Hence, NIWA assume that until about 2050,
climate change in New Zealand will be similar to that
which occurred during the 20th century (i.e. dominated
by natural climate variability), and any climate change
associated with AGW will predominantly occur later.
So far this century, the measured global air surface
temperature and sea level are “rising” at a much lower
rate than predicted (New Zealand has cooled by 0.2°C
and sea level has fallen 3 cm).
Alternatively, based on ocean-atmosphere
interactions, it is possible to predict future climate,
because the spiciness anomalies that contribute to
climate change already exist in the oceans. Therefore,
they can be measured and it is possible to predict their
eventual re-emergence. The available data indicate that
a negative PDO is established, and the AMO is also
switching to a negative mode in the Atlantic Ocean. This
has been contributing and will continue to contribute to
global cooling for another 20-25 years. As the oceans
cool, global sea levels should fall although continued
retreat of glaciers will slow the drop.
New Zealand will experience a climate consistent
with a negative PDO, probably most like the pattern that
existed between the mid 1890s and 1922 with relatively
weak ENSO uctuations. This will mean more intense
rainfall events in some areas, particularly the upper
North Island, increased frequency and magnitude
of storm surges, and more frequent waterspouts and
mesoscale storms. Drought frequency is likely to
increase for most parts of the South Island. Snowfall is
likely to increase in the North Island and eastern parts
of the South Island, and decrease for the western side of
the Southern Alps.
After 2030, there should be a switch to a positive
PDO. It is unclear whether this will result in warming.
During the 20th century, increasing solar activity and
decreasing low level clouds have contributed to an
increase in ocean heat content. Since the solar maximum
in Cycle 23, solar activity has decreased and ocean heat
content has also been falling. Extrapolation of known
solar cycles (particularly the Gleissberg and Seuss
cycles) predicts solar activity will continue to decline
this century. If this is correct, then global climate is
moving towards another Little Ice Age.
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