The Oceans and Global Climate Change
Andrew J. Weaver
School of Earth and Ocean Sciences
University of Victoria
PO Box 3055, Victoria, BC, V8W 3P6, Canada
Tel: (250) 472-4001
Fax: (250) 472-4004
email: weaver@uvic.ca
Abstract
The Intergovernmental Panel on Climate Change (IPCC) released its second scientific assessment of
climate change in 1996. Central to the findings of this study is the statement that “the balance of evidence
suggests a discernible human influence on global climate”. This statement arose from recent studies that
compared the observational record over the last century with the results from numerical modelling
simulations of the climatic response to increasing greenhouse gases and anthropogenic sulphate
aerosols. It has profound implications for policy makers as it is the first time that IPCC has stated that
global warming has been detected in the observational record. Modelling studies reported in the IPCC
second assessment also suggest “best estimates” of 2 C warming and 50 cm sea level rise by the year
2100, relative to 1990, with amplification of the warming at high latitudes and over the interior of
continents. Similar IPCC “best estimates” of emission scenarios suggest that the atmospheric
concentration of carbon dioxide will continue to increase over the next few centuries. Stabilization of
atmospheric carbon dioxide levels, even as high as four times preindustrial levels, require emission levels
sunstantially below those of 1990. Herein, the observational record is briefly reviewed, as are the recent
modelling results that have lead to the statements above. By appealing to the paleoclimatic record for
analogies of a climate warmed through anthropogenic greenhouse gases, a discussion of potential
climatic swings and regime changes is presented. The policy implications of this work are also addressed.
CO2 and Climate  Past and Present
As governments around the world move towards ratification of the Kyoto Accord, potential climate change
associated with increasing greenhouse gases and tropospheric aerosols has moved to the forefront of
public, corporate and political attention. Greenhouse gases act to trap outgoing infrared radiation that the
earth emits to space and reradiate this energy back to the Earth's surface, thereby warming it until a new
equilibrium is reached. Anthropogenic aerosols (such as sulphates) are released to the atmosphere in
highly industrial areas (e.g., eastern USA, western Europe, southeast Asia). These tiny liquid or solid
particles act to scatter incoming solar radiation back to space and hence cool the planet. They may also
indirectly affect climate by promoting the formation of clouds. Any change in the atmospheric
concentration of greenhouse gases or aerosols therefore acts to perturb the radiation balance at the top
of the atmosphere. This change in net radiation then acts as a radiative forcing (Figs. 1, 2 [from IPCC,
1996]) which drives the Earth’s system until a new equilibrium climate is reached. In this new radiative
equilibrium the Earth gains as much energy from the sun as it emits to space.
It is well known that in the past, warm climates were associated with high levels of atmospheric CO 2 and
cold climates with low levels of CO2 (Fig. 3). For example, during the last glacial maximum ~21,000 years
ago, atmospheric CO2 levels were about ~200 ppm (compared to ~365 today and ~280 at 1850). Low
atmospheric CO2 levels prevailed through glacial times, increasing for short periods during interglacials.
What is less clear, and an aspect which is hotly debated, is whether changes in CO2 in the past lead or
lagged changes in global mean temperature. While an academically interesting debate, it is in itself not
terribly relevant. During a transition from a warm to a cold climate (or vice versa) the Earth system is no
longer in radiative equilibrium. The existence of anomalously high [low] CO 2 (or other greenhouse gas)
levels is necessary to let the system remain in equilibrium with warmer [colder] surface temperatures. As
such, whether changing temperatures somehow caused changes in CO2 levels in the past, or whether the
converse occurred, is not as important as realising that at equilibrium, throughout the geologic record,
warm climates are high CO2 climates and cold climates are low CO2 climates with few exceptions.
Andrew J. Weaver
Andrew J. Weaver
a)
b)
Figure 3: a) CO2 (bottom), CH4 (middle) and estimated temperature changes (top), during the past
160,000 years as obtained from the Vostok ice core in Antarctica (Barnola et al., 1987; Chappelaz et al.,
1990); b) 35,000 year record of Antarctic (curve A) and Greenland (curve B) isotope temperatures;
Greenland ice core CH4 (curve C); Antarctic ice core CO2 (curve D); South Atlantic foraminiferal 13C
(curve E) (from Sowers and Bender, 1995).
The best estimates of future anthropogenic emissions of CO2 (IPCC 1992) suggest that atmospheric
concentrations of CO2 will continue to rise well into the future with a best guess estimate of ~700 ppm by
the year 2100 (Fig. 4 [from IPCC, 1996]). If the projected year 2000 emissions are held fixed, this reduces
to ~500ppm by year 2100, although equilibrium has not been reached.
IPCC (1995,1996) also presented various scenarios of atmospheric CO 2 concentration leading to
stabilization in the future (Fig. 5 [from IPCC, 1996]). From these assumed curves, the implied
anthropogenic CO2 emissions required to reach this stabilization were calculated (Fig. 6 [from IPCC,
1995]). Also shown is the “best estimate” of anthropogenic emissions to the year 2100 as produced by
IPCC. These results provide the very shocking realization that if we wish to stabilize levels of atmospheric
CO2 even at levels which are nearly four times (not shown) the preindustrial value of 280 ppm, future
emissions must drop well below those of 1990. The problem of reducing emission levels is compounded
by the reality of human population growth: if we wish to cut our total emissions to one half 1990 levels by
the year 2100, and if global population (Fig. 7) doubles by that time, then per capita, emissions must drop
by one quarter. This does not account for the issue of equity for developing nations wishing to attain the
same standard of economic well being as in the developed nations.
Figure 7: Human population density.
Andrew J. Weaver
Observational Evidence of Climate Change
This section is purposely brief and only highlights three pieces of direct evidence indicating climate
change over the 20th century. The first indicator is the now well know global mean temperature curve (Fig.
8) which suggests about a 0.6C warming over the last century. Missing from this figure are the years
1995, 1997, and 1998 representing the third, second and first warmest years on record, respectively.
Recently, Mann et al (1999) analysed tree ring data around the globe to provide a comprehensive
reconstruction of northern hemisphere average temperatures during the last 1000 years. Figure 9 clearly
reveals the general northern hemisphere cooling trend which occurred over the last 1000 years,
interrupted by a dramatic increase in northern hemisphere temperature at the turn of the 20th century. In
the 1000 year record, 1998 represented the warmest year, 1990s the warmest decade and the 20 th
century the warmest century.
Figure 9: Northern hemisphere temperature reconstruction (represented as an anomaly from the 19021980 mean) based on tree ring and ice core records. Data redrawn from Mann et al. (1999).
The third indicator (Fig. 10  from Stone, 1999; Stone et al. 2000) demonstrates a significant 20th
century trend towards increasing Canadian precipitation, especially at high latitudes. A summary of some
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additional evidence is presented in Fig. 11 (from IPCC, 1996).
Figure 10:Trends in regional & district precipitation. Values are in %/decade, and uncertainties are 95%
confidence limits. Trends significant at the 5% level (Stone, 1999; Stone et al. 2000) are indicated in bold.
Figure 11: Summary of 20th century climate change evidence (from IPCC, 1996)
Andrew J. Weaver
Simple Model Projections of Future Climate and the Impact of Kyoto
The most recent projections of global mean temperature and sea level increase over the next century, as
issued by the IPCC are shown in Figs. 12a and b (from IPCC, 1996), respectively. These suggest “best
estimates” of 2°C and 50 cm for global mean temperature increase and sea level rise, respectively, by the
year 2100. For low and high climate sensitivities, it has been suggested that 1°C–3.5°C (Fig. 12c [from
IPCC, 1996]) and 15-95 cm (Fig. 12d [also from IPCC, 1996]) provide an estimated range of anticipated
increases in global mean temperature and seal level by 2100.
Andrew J. Weaver
Wigley (1998) conducted a series of experiments in an attempt to understand how the curves in Fig. 12
were affected by emission targets set through the Kyoto Accord. Under a variety of post Kyoto emission
scenarios (Figs. 13, 14 [both taken from Wigley, 1998]) he noted that its implementation would have little
effect on 21st century atmospheric CO2 levels and hence projected global mean temperature and sea
level increases. For example, the consequences of maintaining the Kyoto emission targets, with no
further reductions, suggests its effect would be to diminish global warming by <0.1C (from 2.0 to >1.9C)
by 2100. Nevertheless, the Kyoto Accord is an important first step towards implementing policy required
to reduce future anthropogenic emissions of CO2.
General Circulation Model Projections of Future Climate
The most recent and sophisticated coupled climate models, which include both observed anthropogenic
greenhouse gas increases and the direct effect of sulphate aerosols, are now able to simulate the
observed global mean temperature increase since 1860 (Fig. 15 [from IPCC, 1996]), but they have more
difficulty representing its variability. The inclusion of the direct effect of sulphate aerosols is important
since the radiative forcing associated with 20th century greenhouse gas increases alone tends to
overestimate the 20th century warming. There are still questions concerning the known unknowns such
as the parameterization of subgrid-scale processes within these coupled models (e.g., clouds,
hydrological cycle, air-sea exchange, ocean mixing). Nevertheless, this is an important result if one is to
have confidence in the ability of these coupled models to project future climate changes.
These same models have all shown a trend towards increasing global precipitation, with an intensification
of the signal at the high northern latitudes, consistent with the observational record. In the high latitudes
of the northern hemisphere, the climate projections further suggest an amplification of the warming (Fig.
16 [from IPCC, 1996]) due to the reduction of sea ice cover and the accompanying decrease in surface
albedo (amount of incoming solar radiation reflected back to space). In the region around Antarctica, very
little change (or even slight cooling) is projected over the next few centuries (depending on the coupled
model), due to the relatively efficient absorption of heat by the ocean there. This latter result has been the
subject of some controversy as certain models, that include a more realistic treatment of ocean mixing, do
not find such an asymmetrical response in the Southern Ocean.
Andrew J. Weaver
Figure 16: Seasonal change in surface air
temperature from 1880-1889 to 2040-2049 from
the MPI coupled model (Germany) [IPCC, 1996].
Climate Change Detection and Attribution Studies
As noted earlier, coupled models have a general tendency to underestimate the variability found in both
the instrumental and palaeo proxy record, especially over the oceans (the converse is true over land
when compared to the instrumental record). Most coarse-resolution coupled models do not capture the
processes involved in El Niño well, which is not surprising as neither the equatorial ocean wave guide nor
localized atmospheric convection are well resolved. In addition, none of the aforementioned simulations
examine the potential effects of stratospheric aerosols associated with volcanic emissions. On the longer
timescales, none of the aforementioned studies included variability in solar forcing. Cubasch et al. (1997)
demonstrated that when solar variations are included in the ECHAM3/LSG model, their simulation from
1700 through the 20th century showed enhanced low frequency variability associated with variability in
solar irradiance. Nevertheless, both they, Hegerl et al (1997) and Tett et al. (1999) show that the
observed temperature change since 1945 is largely caused by increasing greenhouse gases and
sulphate aerosols with little contributions from changes in solar irradiance. As noted by Tett et al. (1999),
however, solar irradiance changes could have contribute to the warming in the early part of the century.
Both these studies show that the recent climate warming is both detectable and attributable to increasing
anthropogenic greenhouse gases and sulphate aerosols (> 95% level). Of paramount importance is the
recent statement issued by IPCC (1996) regarding the detection of a global warming signature in the
present climate record. This statement reads:
“The balance of evidence suggests a discernible human influence on global climate”
Thus, for the first time it appears that there is a consensus view that global warming has begun. Stronger
statements are expected to arise from the IPCC Third Scientific Assessment report now being drafted, as
the body of evidence concerning the detection and attribution of climate change grows.
Figure 17 shows the Taylor and Penner (1994) annual mean surface air temperature difference between
atmospheric general circulation model integrations under present-day and preindustrial CO2, and sulphate
aerosol conditions. The observational record (e.g., Fig. 18 [from IPCC 1996]) was then analysed by
Santer et al., (1995) to determine whether the expected model pattern of change could be seen to
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emerge over time within this observational record. This represents only one of the now many attempts to
detect climate change over the 20th century and attribute it to anthropogenic forcing.
Andrew J. Weaver
The Climate of the Last 140,000 Years  The Possibility of Surprises
One of the most important indicators of past climates/climate variability is the record from the Greenland
ice cores (Fig. 19). These ice core records clearly show the slow transitions between the glacial and
interglacial periods (Fig. 20) thought to be due to changes in the latitudinal and seasonal distribution of
solar radiation received by the Earth (Milankovitch theory). That is, the glacial/ interglacial cycles are
believed to be associated with changes in the precession of the Earth’s axis (~23,000 year period),
variation in the Earth’s axial tilt (~41,100 year period), and changes in the eccentricity of the Earth’s orbit
around the sun (~ 100,000 year period). Early analyses of these ice cores have further shown that the
climate of the North Atlantic during the last glaciation was marked by abrupt swings between relatively
warm and relatively cold regimes. These climate shifts were observed to occur very rapidly (in several
decades) and quite frequently (every several thousand years or so). Similar oscillations have been found
during the transition between the glacial and interglacial climates, the most recent of which is the Younger
Dryas event which ended about 10,000 years ago. This event concluded the transition from the last iceage to the present interglacial period (known as the Holocene). While it has long been known that the
climate of our present Holocene has been marked by much weaker natural climate variability on both the
century and decadal timescales, we have not observed any rapid and dramatic climate shifts (Fig. 21).
Figure 19: Map of Greenland showing the location of the Camp Century, Dye 3 and Summit ice core site.
Recent findings from the Greenland ice core project (Grip, 1993; Dansgaard et al., 1993  Fig. 22) have
added a new twist to the global warming debate. It has been known for a long time that the glacial periods
and the transitions to/from these glacial periods were marked by intense climate swings (regime changes)
which occurred over very rapid times (order decades). The last such cooling event, the Younger Dryas
Event, ended about 10,000 years ago. This event concluded the transition from the last ice-age to the
present interglacial period (known as the Holocene). Prior to the publication of these recent ice core
findings, the existing coarse temporal resolution paleoclimatic data suggested that interglacial periods
were free of such intense climate variability. What the Greenland ice core project has revealed is that the
climate of the last interglacial (referred to as the Eemian period — 115,000 to 135,000 years ago) may
have been characterized by three climatic states. A state very much like today's climate and states
significantly warmer and colder than today's climate. It was also found that the transition between these
states occurred very rapidly and that the climate system never remained in one state for more than about
2,000 years (we have been in a stable climatic state for about 10,000 years). While there is some concern
as to the reliability of the ice core data corresponding to the latter half of the Eemian (GRIP, 1993; Taylor
et al., 1993; Grootes et al., 1993), our understanding of interglacial climates has nevertheless suddenly
Andrew J. Weaver
been questioned: It was thought that interglacial climates were relatively stable — now it appears that it is
only our present interglacial which has a stable climate and we are left with the question of why this is so.
Modelling results (Weaver and Hughes, 1994; Tang and Weaver, 1995) have provided us with a possible
answer. The Eemian interglacial was, on average, slightly warmer than the present Holocene interglacial.
Associated with this warmer atmosphere would be enhanced low latitude evaporation and, through
poleward atmospheric water vapour transport, enhanced high latitude precipitation. This enhanced
hydrological cycle would cause a destabilization of the ocean's conveyor belt (thermohaline) circulation
which could then rapidly, and unpredictably, switch from one mode of operation to another, with
catastrophic changes in northern hemisphere climate. Thus, analogous to the porridge of Goldilocks and
the Three Bears, perhaps the present climate is stable as it is neither too cold nor too hot — it is just the
right temperature.
Summary
In conclusion, we note that the Eemian interglacial may offer us a glimpse as to the type of rapid climate
variability which we might expect in a warmer climate. If we continue to increase our emissions and
simply maintain a policy of adaptation then we may be in for a big surprise. To reduce the likelihood of
surprise, policy makers should not only be focusing on adapting to a warmer climate but should be acting
to ensure that in the long run substitutes for fossil fuels are found that are adequate to meet our energy
requirements while reducing greenhouse gas emissions. It is not so much the new warmer mean climatic
state with which we must concern ourselves, but rather the potential variability associated with this new
state. But even the choice of action involves hard decisions. For example, it could even turn out that there
is already enough anthropogenic effects in the pipeline to set off climatic regime changes. If so, costly
Andrew J. Weaver
attempts now to prevent further emissions would be irrelevant: it would be much more sensible to invest
resources in means of enabling most of humanity to withstand rapid regime change, albeit, at what may
turn out to be a substantially reduced average level of economic well-being.
Figure 22: The oxygen isotope (18O o/oo) record obtained from the GRIP Summit ic core over the last
150,000 years. A change in oxygen isotope of 1 o/oo corresponds to about a 1.5°C temperature change
in the region of the North Atlantic. Redrawn from GRIP (1993).
References:
Barnola, J.M., D. Raynaud, Y.S. Korotkevitch, and C. Lorius, 1987: Vostok ice core: A 160,000 year
record of atmospheric CO2. Nature, 329, 408-414.
Chappellaz, J., J.M. Barnola, D. Raynaud, Y.S. Korotkevitch, and C. Lorius, 1990: Ice-core record of
atmospheric methane over the past 160,000 years. Nature, 345, 127-131.
Cubasch, U., R. Voss, G.C. Hegerl, J. Waszkewitz, and T.J. Crowley, 1997: Simulation of the influence of
solar radiation variations on the global climate with an ocean-atmosphere general circulation
model. Climate Dynamics, 13, 757–767.
Dansgaard, W., S.J. Johnsen, H.B. Clausen, and C.C. Langway, 1970: Climatic record revealed by Camp
Century ice core. In: The Late Cenozoic Glacial Ages, Turekian, K.K. (Ed.), Yale, pp. 37-56.
Dansgaard, W., S.J. Johnsen, H.B. Clausen, D. Dahl-Jensen, N. Gundestrup, C.U. Hammer, and H.
Oeschger, 1984: North Atlantic climatic oscillations revealed by deep Greenland ice cores. In:
Climate Processes and Climate Sensitivity, Hansen, J.E., and T. Takahashi (Eds.), AGU
Monograph, 29, 288-298.
Dansgaard, W., S.J. Johnsen, H.B. Clausen, D. Dahl-Jensen, N.S. Gundestrup, C.U. Hammer, C.S.
Hvidberg, J.P. Steffensen, A.E. Sveinbjörnsdottir, J. Jouzel and G. Bond, 1993: Evidence for
general instability of past climate from a 250-kyr ice-core record. Nature, 364, 218–220.
GRIP Project Members, 1993: Climate instability during the last interglacial period recorded in the GRIP
ice core. Nature, 364, 203–207.
Grootes, P.M., M. Stuiver, J.W.C. White, S. Johnsen and J. Jouzel, 1993: Comparison of oxygen isotope
records from GISP2 and GRIP Greenland ice cores. Nature, 366, 552–554.
Hegerl, G.C., K. Hasselmann, U. Cubasch, J.F.B. Mitchell, E. Roeckner, R. Voss, J. Waszkewitz, 1997:
Multi-fingerprint detection and attribution analysis of greenhouse gas, greenhouse gas-plus-aerosol
and solar forced climate change. Climate Dynamics, 13, 613-634.
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Intergovernmental Panel on Climate Change, 1992: Climate Change 1992: The Supplementary Report to
IPCC Scientific Assessment, Cambridge University Press, Cambridge, UK, 200pp.
Intergovernmental Panel on Climate Change, 1995: Climate Change 1994: Radiative Forcing of Climate
and an Evaluation of the IPCC IS92 Emission Scenarios, Cambridge University Press, Cambridge,
UK, 340pp.
Intergovernmental Panel on Climate Change, 1996: Climate Change 1995: The Science of Climate
Change. Contribution of Working Group I to the Second Assessment Report of the
Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, 572pp.
Mann, M.E., R.S. Bradley, and M.K. Hughes, 1999: Northern hemisphere temperatures during the past
millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters, 26, 759-762.
Santer, B.D., K.E. Taylor, T.M.L. Wigley, J.E. Penner, P.D. Jones, and U. Cubasch, 1995: Towards the
detection and attribution of an anthropogenic effect on climate. Climate Dynamics, 12, 77-100.
Sowers, T. and M. Bender, 1995: Climate records covering the last deglaciation. Science, 269, 210-214.
Stocker, T.F., 1989: Review of climatic fluctuations on the century timescales. CRG Report No: 89-7,
McGill University, 69pp.
Stone, D.A., 1999: An analysis of Canadian precipitation and its intensity. MSc thesis, University of
Victoria, 94pp
Stone, D.A., A. J. Weaver and F.W. Zwiers, 2000: Trends in Canadian precipitation intensity.
Atmosphere-Ocean, in press.
Tang, B. and A.J. Weaver, 1995: Climate stability as deduced from an idealized coupled atmosphereocean model. Climate Dynamics, 11, 141–150.
Taylor, K.C., C.U. Hammer, R.B. Alley, H.B. Clausen, D. Dahl-Jensen, A.J. Gow, N.S. Gundestrup, J.
Kipfstuhl, J.C. Moore and E.D. Waddington, 1993: Electrical conductivity measurements from the
GISP2 and GRIP ice cores. Nature, 366, 549–552.
Taylor, K.E., and J.E. Penner, 1994: Response of the climate system to atmospheric aerosols and
greenhouse gases. Nature, 369, 734-737.
Tett, S.F.B., P.A. Stott, M.R. Allen, W.J. Ingram, and J.F.B. Mitchell, 1999: Causes of twentieth century
temperature change. Nature, 399, 569-572..
Weaver, A.J., and C. Green, 1998: Global climate change: Lessons from the past  policy for the future.
Ocean & Coastal Management, 39, 73-86.
Weaver, A.J., and T.M.C. Hughes, 1994: The ocean as an internal source for rapid interglacial climate
fluctuations. Nature, 367, 447–450.
Wigley, T.M.L., 1998: The Kyoto Protocol: CO2, CH4 and climate implications. Geophysical Research.
Lettters, 25, 2285-2288.