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The Earth's climate changes in response to both variations within the global climate system and to natural and human caused factors that are outside the global climate system. To measure climate trends, scientists work backwards to analyze climate changes that took place a long time ago, and this requires good detective work.

Since the Earth first formed an atmosphere, the climate has been changing. To try to estimate the impacts of climate change, scientists look at past weather patterns, short-term weather predictions, and long-term climate models. Although instrument records have only been kept since the 1860s, scientists can look at past climates through techniques of paleoclimatology. By studying ice cores, tree rings, sediments and other indirect measurements they can, more or less, explain the natural causes of historical climate trends. They believe that Ice Ages and interglacials (cool to warmer periods) are, in part, caused by slow "wobbles" in the Earth's axis and its orbit around the sun. These wobbles affect the amount of energy the Earth receives from the sun and where the energy is absorbed. Through the study of ice cores, scientists can also make assumptions about how the levels of atmospheric greenhouse gases influence ice age cycles. 26

Overall, the climate seems to have been remarkably stable since the last ice age ended some 10,000 years ago. The Earth's average temperature has not varied more than 1°C, since agricultural societies appeared. In fact, we seem to be living in a rather peaceful interglacial compared to the extreme and rapidly changinginterglacial period that took place 100,000 years ago. 27

We do know that the climate has varied in the past and there is no longer any doubt that the Earth's average temperature is rising. Measurements show that the Earth's average temperature has increased by 0.4-0.8°C since the 19th century, and historical data indicate with some certainty that CO2, the major long-lived greenhouse gas, is likely at its highest levels for 20 million years. The Third Assessment Report (2001) by the Intergovernmental Panel on Climate Change (IPCC) concluded that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year. 28 The IPCC further notes that in the last 100 years, snow cover and ice extent have decreased, with a widespread retreat of mountain glaciers in non-polar regions, and northern hemisphere spring and summer sea-ice extent has decreased by 10 to 15 per cent since the 1950s.29

Environment Canada 30 reports that the 10 warmest years in global meteorological history have all occurred in the past 15 years, and the 20th century has been the warmest globally in the last 600 years.

According to the Tyndall Centre for Climate Change Research, the average amount of warming in Canada in the 20th century has been about 0.5°C. This is significant given that the global average temperature difference between the last ice age and today is only about 5°C 31 . The IPCC in its Third Assessment Report projects global warming of 1.4 to 5.8°C between 1990 and 2100. This projected range of increases is based on various computer models of climate and a range of scenarios for global greenhouse gas emissions, including scenarios of continued rapid emissions growth and scenarios under which global emissions fall below current levels by 2100.

Enhanced climate change in Canada, with its high latitude, could mean an increase in annual mean temperatures in many regions of between 5 and 10°C. But a key observation based on reliable data from the past 50 years, and one that makes the present situation different from past climate changes, is this: the speed of present climate change and the amount of change that is occurring both appear much greater than normal.

Arctic lakes warming up

New research by scientists studying algae (specifically diatoms) in Canadian Arctic lakes suggests that temperatures there are warmer than they have been for 5,000 years. Algae that require long periods of open, ice-free water are now thriving in many northern lakes where they were rare 150 years ago. Studies of the lake sediments revealed that algae that need warmer conditions were not found in the lakes until the present time. The paper will be published in the journal Arctic, Antarctic and Alpine Research. (Source: The Globe and Mail, April 8, 2003)

Current trends in changing weather patterns being measured around the globe are consistent with what climate scientists predict for climate change when they use the best available global climate computer models. The overwhelming consensus of professional climate scientists is that current trends in weather patterns being measured around the globe cannot be explained without including the effects of emissions of greenhouse gases from human activities. In its Summary for Policymakers, the IPCC says, "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities" (page 10).

In addition to rising average temperatures, scientists are finding changing precipitation patterns and increased frequency and severity of extreme weather events. Click here to view images of damage caused by extreme weather events, including Hurricane Andrew.

The Earth now has many climate change hotspots and the IPCC predicts that humans will continue to influence the climate throughout the 21st century. Several trends have been identified as early warning signs of further global warming, including earlier arrival of spring, spreading disease, coral reef bleaching, shifts in the ranges and populations of animals and plants, and more extreme weather. Evidence for a direct link to long-term climate change can't be confirmed with 100 per cent certainty, but these trends in actual climate measurements and ecosystem effects are fully consistent with what IPCC scientists predict will occur as concentrations of greenhouse gases rise in the atmosphere.

What does the future hold? Projections are that temperatures will continue rising for the next 100 years and scenarios have been developed to help us see what the impact of various levels of greenhouse gases could be. Northern countries such as Canada are already seeing climate changes. Scientists acknowledge that uncertainty remains, but as models improve, our ability to predict climate change and its impacts will also improve. This section considers the impacts of climate change on climate and weather, looking particularly at the following aspects. The ecosystem effects and socioeconomic effects of climate change are described in other sections of this site.

26. Source: ‘The Evidence from Past Climates,' Climate Change Information Kit, United Nations Framework Convention on Climate Change.
27. Source: ‘The Evidence from Past Climates,' Climate Change Information Kit, United Nations Framework Convention on Climate Change.
28. Source: Climate Change 2001, The Scientific Basis, IPCC: Climate Change 2001
29. Source: Climate Change 2001, The Scientific Basis, IPCC: Climate Change 2001
30. Source: Government of Canada
31. Source: Hengeveld. H (1995), Understanding Atmospheric Change, Second Edition, Environment Canada, pg. 18