Science of Climate Change

FREQUENTLY ASKED QUESTIONS ABOUT THE SCIENCE OF CLIMATE CHANGE

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D. Predicting Climate

D.1 How much is the earth expected to warm in the future?

Response: Without coordinated global action to reduce greenhouse gas emissions, the global average surface temperature relative to 1990 is expected to rise by between 1.4 and 5.8ºC (about 2 to 10ºF) by the year 2100. Even if greenhouse gas concentrations are stabilized, temperatures will continue to rise for centuries after stabilization because of the delay in ocean response.

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Explanation:This is the current best estimate based on the probable range of projected future concentrations of atmospheric greenhouse gases and sulphates (which have a cooling effect) if no specific action is taken to reduce greenhouse gas emissions. The projections also include uncertainties related to climate model performance. Since most greenhouse gases remain in the atmosphere for a long time, the effects of past emissions will persist for centuries even if greenhouse gas emissions from human activities were to stop immediately. It is important to note that temperature changes will occur unevenly around the world. Land will warm more than oceans and greater year­round warming is predicted for high latitudes, with more warming in winter than summer at middle to high latitudes. In Canada, the annual mean temperature could increase between 5 and 10°C over the next century.

Reference: IPCC 2001, WGI, Chapter 9.

D.2 Why is there more than a 4°C range in the amount of global warming projected?

Response: There are two key factors that contribute to the range of estimates. The first is uncertainty about future human behaviour and how these may affect concentrations of greenhouse gases and aerosols in the atmosphere. The second is scientific uncertainty about how fast and how much the climate system will be affected by changes in these concentrations.  When combined, these show a range of more than 4°C, extending from the most optimistic outcome of a 1.4°C warming by 2100 to the most pessimistic outcome of 5.8°C warming.

Explanation: Future emissions of greenhouse gases and aerosols will depend on how rapidly human populations and economies will grow in future decades, how efficiently societies will use energy, the type of energy they use and how human use of land is likely to change.  These are uncertainties about future social behaviour, rather than about the climate system. However, there also remains considerable uncertainty about how the climate system will respond to changes in concentrations of greenhouse gases and aerosols. During the next few decades, the latter is the more important, while the former is the primary source of uncertainty in the second half of the century.  The 4°C range in projected warming for 2100 arises when these uncertainties are combined in model simulations.

D.3 How are we to believe the results of climate models when their various forecasts for future climate differ so much? 

Response:  While the models disagree on the details of future climate change, there is actually good general agreement on the continental scale pattern and significance of expected future changes in temperature, particularly over the next few decades. 

Explanation: Various models use alternative techniques for describing how different components of the climate system function. Furthermore, there is significant natural variability within the climate system, so that identical experiments with the same model can be expected to show different details in their results.  Hence there are some significant differences between model experiments on the details and rate of future climate change. However, all models agree that the warming will be significant and likely unprecedented in human history, that continents will warm more than oceans, that high latitudes will warm more than low latitudes, that sea levels will rise, snow and sea ice extent and thickness will decrease, and that there will be an increase in average global precipitation, accompanied by major changes in the distribution of precipitation. 

D.4 How reliable are the models used to predict future climate change?

Response: Climate modellers use the most advanced physics and mathematics available today to develop complex climate models. Models are first tested against observed climates and climates of the past to ensure they can adequately simulate real climates.  Once they have past these and other tests, they are used to project future climates for various scenarios of future greenhouse gas and aerosol emissions. While these tests show significant disagreements with observed and past climate data at regional scales, the advanced models of today can replicate the global pattern and trends quite well. Hence, modelers are confident that they can provide useful indicators of how the climate will respond to continued human interference with the climate system.

Explanation: The computer climate models used to predict future climates are based on well-accepted physical principles of science and a wealth of scientific observations of the climate system. Complex mathematical equations are used within the models to describe how these principles affect the interactions of land, sea, ice and air, which together determine the Earth's climate. These models are then operated on very large computers to simulate how the climate behaves, starting from a stand still. Models are first tested to see how well they can describe today's climate, and most are now able to describe the main features of the climate system quite accurately. There are, however, significant regional differences apparent in most models, both because the resolution of the models is too coarse to capture all the important regional interactions of the climate system and because some of these interactions are as yet inadequately understood. The models are then also run for climates of the past, including the last 100 years, the peak Holocene climate of 6000 years ago, and the last glacial maximum of 18,000 years ago. Most advanced models now also simulate these quite well, particularly for the past 100 years. Finally, there are also model inter-comparison studies that seek to understand where and why the model results differ. Over the past four decades of climate model evolution, the confidence in their performance has improved immensely. Hence, while there are still significant uncertainties in model performance, there is consider confidence that they can help provide useful advice on future climate change.

D.5  Models used for weather forecasting often can't even properly predict the weather for the next few days.  How can we expect credible predictions from climate models for decades and even a century into the future?

Response: Climate is average weather, which is more predictable than day-to-day and hour-to-hour weather changes. Weather behaviour is chaotic and often difficult to predict beyond a week or so into the future. By comparison, climate is largely determined by global and regional geophysical processes that change slowly. Hence, if these factors are properly understood and predictable, then the climate can be forecasted far into the future with some confidence.

Explanation: Day-to-day local weather is largely determined by atmospheric circulation and the formation of large scale weather systems. Because of the chaotic nature of the atmosphere, predictability decreases with time, and is quite poor beyond a few weeks into the future. Climate, on the other hand, represents average weather and its expected variability. These are determined by factors such as incoming solar radiation (which varies with latitude and time of year), the influence of prevailing characteristics of cloud cover, aerosols and other components of the atmosphere on the flow of the sun's energy into the atmosphere and of heat energy out again, prevailing winds and other atmospheric conditions, and local geophysical conditions that, in general, change slowly and in a more predictable manner. Thus, while forecasters would be unable to predict day-to-day weather six months into the future, they can provide good approximations of the changes in seasonal climates because of known physical processes that cause conditions to change from winter to summer and back again. They can also provide estimates of the changes in probability of different kinds of weather events, such as sub-zero minimum temperatures, maximum temperatures in excess of 30°C, snow blizzards or thunderstorms. Likewise, climate models, when looking much farther into the future, project how the climate characteristics, averaged over several decades, might change in response to projected changes in the factors that determine the climate.

D.6 Don't the comparisons between climate observations and predictions by computer models suggest that models exaggerate global warming?

Response: No. as Shown in Figure C.3, model simulations conducted in recent years that include all of the key factors affecting climate over the past century show very good agreement with observations.  Hence there is no evidence that the model projections for global warming are biased one way or the other.

Explanation: A decade ago, when most model experiments only included the climatic effects of rising greenhouse gas concentrations, the warming rates simulated for the past century often exceeded those observed. However, recent experiments that also include the more complex effects of other human activities (such as emission of aerosols and the depletion of the stratospheric ozone layer) and of natural factors (solar changes and volcanic aerosols) produce simulations of long-term temperature trends that agree well with those derived from observations. This suggests that the models do not exaggerate the sensitivity of climate to human influences.

D.7 Earlier IPCC estimates of global warming for the 21^st century ranged from 1.0°C-3.5°C. In the IPCC Third Assessment Report, the range of projected warming actually increased to 1.4°C-5.8°C.  If global climate models are becoming more sophisticated, why is the scientific uncertainty increasing? 

Response: The primary reason for the increase in the range of estimates for future climate change presented during the last IPCC assessment was the use of new emission scenarios for greenhouse gases and aerosols. These scenarios suggest a larger range in possible human induced emissions than projected in earlier emission scenarios. The increase in projected temperature range is therefore primarily due to demographic factors, not increased scientific uncertainty.

Explanation: For the first IPCC assessment, experts focused on the use of equilibrium climate models to predict that, once the climate system has fully responded to a world with twice as much CO₂ as today, the average surface temperature would be about 1.5 to 4.5°C warmer than today. Although they also presented some preliminary results from coupled climate models forced by hypothesized business-as-usual emission scenarios, these results did not reflect the full range of uncertainties about future emission rates. During the Second Assessment, a series of six business as usual scenarios (known as IS92 scenarios) were used to approximate the range of emissions due to human activities to 2100, and the cooling effects of increasing concentrations of sulphate aerosols were added into model simulations. The results suggested a possible range of warming by 2100 of 1.0 to 3.5°C. In the Third Assessment Report (TAR), completed in 2001, a new set of SRES emission scenarios, believed to be more representative of the range of future human activities without policies specifically aimed at reducing risks of climate change, was used for future projections. These new scenarios had a slightly larger range in emissions between 1990 and 2100 than the older IS92 scenarios. They also reduced the magnitude of the offsetting effects of aerosols used in the IS92 scenarios, noting that local air pollution concerns would force nations to take action to curtail aerosol emissions. These changes in emission scenarios were the primary reason for the enhanced range for the TAR climate change projections. Hence they reflect increased uncertainties about future human behaviour, not increased uncertainties in climate models. In fact, the range of scientific uncertainty in climate model projections of how the climate will respond to a specific emission scenario has not changed significantly. While advanced models do reflect a significant improvement in understanding of the climate system, there continue to be major challenges in accurately describing some aspects of the climate system in models, which are still limited by available computing power.

D.8  Why are estimates for future sea level rise under various global warming scenarios becoming progressively lower as new research results emerge?

Response:  The slightly lower estimates for sea level rise by 2100 provided in the more recent IPCC assessments occur for two reasons. First, the offsetting effects of aerosols on rates of climate warming and hence rates of sea level rise were not included in the in the First Assessment Report, but were in the second and third assessments. Secondly, improvements in the understanding of how oceans and ice sheets will respond to warmer climates suggest that the rate of heat uptake by these systems from the atmosphere may be slower than previously thought. However, while these improvements have lowered projections for sea level rise within the next century, they do not significantly alter the projections for full long-term response of sea level rise to human interference with the climate over subsequent centuries. 

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Explanation: Estimates for sea level change as a result of global warming continue to cover a broad range of values.  In the first IPCC assessment (1990), sea level rise by 2100 was estimated at a possible range of between 30 and 100 cm.  In its Second Assessment, IPCC lowered this estimate to a range between 15 cm and 95 cm, primarily because of the inclusion of the masking effect of aerosols in estimated rates of surface climate change.  The Third Assessment in 2001 revised this slightly to a range of 9 to 88 cm, reflecting improved estimates of ocean and ice sheet inertia. The largest uncertainties in these estimates relate to the role of the Greenland and Antarctic ice sheets.  The Greenland ice sheet currently appears to be decreasing in volume, and is therefore an apparent source of water for sea level rise.  It is expected to continue to do so, but the contribution to sea level rise is very sensitive to changes in precipitation, melting and ice sheet dynamics. In contrast, while ice shelves along the Antarctic Peninsula show signs of disintegration, the Antarctic ice sheet itself may be slowly building due to a moister climate.  The combined changes in ice sheets appear to be secondary to the contributions to rates of sea level rise from direct thermal expansion of oceans and melting of temperate glaciers as they warm.  Because oceans and ice sheets respond very slowly to changes in climate, sea levels will continue to rise for centuries after surface climates have already stabilised.  Hence sea level rise continues to present a serious future threat of catastrophic inundation of coastal and island states.

Reference: IPCC 2001, WGI, Chapter 11.

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