Key Findings1 Atmospheric Composition And Radiative Forcing Atmospheric CO2 concentrations continue to rise, and appear to be unprecedented in at least the past 880,000 years.
The atmospheric concentration of CO2 reached about 386 ppm in 2008, 0.5% above that for the preceding year and 37% higher than pre-industrial levels. The rate of increase since 2000 has been about 30% more rapid than that observed in the 1990s (2.1.1).
Since 2005, emissions from fossil fuel combustion have increased by 3.3% per year, which is at the high end of the IPCC SRES scenarios. These emissions are the lead cause for the increased growth rate in atmospheric CO2, although, a recent decline in land and ocean sinks also appears to be a factor (2.1.1).
The role of Canadian forest ecosystems as carbon sinks has virtually disappeared. Key factors have been the increase in wild fire losses, and the outbreak of mountain pine beetle infestation in western forests (2.1.2).
Although the fertilization effect of increasing atmospheric CO2 has been an important factor in the land carbon sink in recent decades, the magnitude of this effect may have been significantly overestimated (2.1.2).
Concentrations of other long-lived greenhouse gases also continue to rise.
The atmospheric concentration of methane was 1789 ppb in 2007 – a level about 156% higher than pre-industrial levels (~ 1750). Ice core data indicate that this level is without precedence during the past 880,000 years (2.2.1).
There has been no significant trend in methane concentrations during the past two decades. It now appears unlikely that the projected concentrations for the next few decades in most of the IPCC SRES scenarios will be achieved. However, concentrations began to rise again in 2007 (2.2.1).
Net anthropogenic climate forcing due to changing atmospheric composition is estimated to be about 2 W/m2, a factor of ten greater than that due to the combined effect of all natural forcings.
Anthropogenic forcing during the past century has been dominated by the radiative effects of rising concentrations of well mixed greenhouse gases (3.5).
There is an increasing focus on quantifying both the direct and indirect radiative effects of short-lived greenhouse gases and aerosols. Indirect effects of ozone on climate through reduced CO2 uptake by vegetation could be as important as its direct effect. The cooling effect of NOx-driven methane depletion appears to slightly exceed the warming effect of NOx enhancement of ozone for a net cooling impact (3.1).
The total indirect cooling effect of sulphate aerosols through increased cloud cover and albedo may be as little as -0.2 W/m2 and hence significantly overestimated in past studies. A key reason for these lower estimates is that that these aerosols, in addition to making clouds more reflective, also change atmospheric circulation and redistribute clouds and precipitation in a manner that, in some regions, can counteract the enhanced cloud albedo (3.2).
Black carbon aerosols have a significant warming influence within the lower atmosphere, particularly in heavily polluted regions. Deposition of black carbon on Arctic snow and ice surfaces is also a major regional climate forcing in spring-time. This influence peaked in the early 20th century, when coal was still the dominant fossil fuel in use. Some of the observed early 20th century Arctic warming may have been due to BC forcing (3.2).
Model Performance and Results Advanced climate system models are now able to replicate observed and past climate changes reasonably well at global and continental scales.
Although models still differ in terms of the magnitude of the water vapour feedback on its own, they now agree quite well on the strong positive feedback caused by the combined water vapour-lapse rate feedback within the climate system. Recent research indicates that this feedback may be greater than previously estimated (4.1.1).
The description of clouds within models has improved significantly. However, the regional and global strengths of the cloud feedback still remain the largest contributor to uncertainty in the projections of the climate system response to future external forcing (4.1.1).
Global surface albedo feedbacks are also demonstrated to be moderately positive and second only to water vapour feedbacks in importance. This feedback is strongly positive in regions where changes in ice and snow cover can dramatically change surface albedo. Carbon cycle feedbacks are recognized as important and largely positive but many important aspects of vegetation-climate feedbacks still need resolution in dynamic vegetation models (4.1.2).
Climate models continue to project large changes in future climate under IPCC SRES emission scenarios.
There is considerable confidence that the global climate sensitivity to radiative forcing associated with a doubling of carbon dioxide concentrations is in excess of 2˚C. Experts are much less confident about the upper end of the sensitivity range but agree the probability that it will be greater than 7˚C is small (4.2.1).
Results in general suggest a 50% probability that global average temperatures will rise another 1.5˚C within the next three to five decades. Warming by 2100 is projected to be in the range of 2 to 4˚C (4.2.3).
Average global precipitation is projected to increase by between 1 and 3% for each degree of surface warming. In general, precipitation and runoff are projected to increase significantly in high latitudes, south-east Asia and central Africa, and to decrease in the subtropics, including the Mediterranean, south Africa, south North America and central America (4.2.3).
The ocean circulation system will slow down under warmer climates, but there is disagreement between model results on the rate and magnitude of the slowdown. The system may shut down completely if the freshwater addition is large. This could occur under high SRES emission scenarios (4.2.3).
Trends in Past and Current Climates Global average surface temperature is now about 1˚C above pre-industrial levels and, at least for most of the Northern Hemisphere, is likely unprecedented in at least the past 1300 years.
The range in long term fluctuations in global average temperature during the past 880,000 years, from glacial periods to interglacials and back again, is now estimated to be on the order of 3 to 7°C. However, temperatures have been relatively stable for the past 10,000 years of the current interglacial (5.1.1).
Reconstructions of climates for the past two millennia, using multiple proxy data and advanced statistical techniques, continue to show that the temperatures of the extratropical region of the Northern Hemisphere during the past half century are very likely unprecedented in at least the past 1300 years, and possibly longer (5.1.3).
Average tropospheric temperatures have also warmed by 0.2°C/decade since 1979, a rate very similar to that at the surface. The upper troposphere may be warming more rapidly, possibly by as much as 0.65°C/decade (5.2.1).
Within Canada, average surface temperatures have increased by 1.2°C since 1955. Winter warming in some northern regions, such as the MacKenzie River Basin, has been as much as 4.7°C over the past 60 years (5.2.1).
Some parts of the world are getting wetter, others are becoming dryer.
There has been an overall global increase in both average precipitation and in soil moisture during the past half century. Largest increases in soil moisture are found in North America (5.2.2).
Increased evaporation can offset increased precipitation. Major droughts still occur with regularity – and increasingly so in some regions. In recent decades, regions of west Africa and south Asia have been among the most significantly affected by such droughts (5.2.2).
Many lakes and ponds in the high Arctic that have been present for millennia are now drying up during summers, causing collapse of aquatic ecosystems within them (5.2.2).
While global net water discharge from rivers into the oceans appears to have increased by almost 8% over the past century, the changes during the past half century appear to be small. This is because large increases in some regions such as northern Europe have been offset by decreases elsewhere. Some mid-latitude rivers show as much as a 60% decrease (5.2.2).
Snow and ice are in retreat around the world, while sea levels continue to rise.
Annual Arctic Ocean ice cover has been declining by 3.4 to 4%/decade since 1979, and by about 10%/decade since 1996. Ice cover reached a record low in September 2007 of 37-38% below the climatological average annual minimum. Less than 5% of the Arctic ice is now 7 years old or older (5.2.3).
More of the old ice from the Arctic Ocean ice pack is now entering the Canadian Arctic straits. As a result, despite the increased melt, there has been little net change in old Arctic ice within these straits over the past 40 years (5.2.3).
Net Greenland ice sheet volume loss in recent years has been at least 100 km3/year, and possibly as high as 249 km3/year. The area of surface melt increased since 1988 at an average rate of about 1.3%/year. Increased snow fall at high elevation partially offsets the enhanced melt but other factors such as increased glacial flow into surrounding oceans contribute to the ice loss (5.2.4).
Contrary to past studies, new analyses indicate that total ice volume in Antarctica has also been decreasing since 1996. One factor is increased melt along the Antarctic ice sheet margins, which is now occurring further inland. This has not been offset by an equivalent increase in interior snow fall. The loss from the West Antarctic sheet is estimated to be about three times that from the much larger East Antarctic sheet (5.2.4).
Most other ice caps and glaciers around the world are also melting at unprecedented rates. Some of the greatest losses are in Europe, where total ice volume decrease since 1850 is estimated at more than 60%, and in New Zealand, with decreases of about 48% (5.2.4).
The estimated 17-20 cm rise in global sea levels during the 20th century is about three times that of the preceding century (5.2.4).
Spring snow cover extent across the Northern Hemisphere has decreased by 1.28 million km2 over the past 35 years. Meanwhile, total permafrost area across Canada may have declined by 5.4% since mid-19th century, and the active layer above the permafrost increased by about three-quarters of a meter (5.2.4).
Many aspects of global weather have become more extreme in recent decades.
Hot summers have become more frequent in almost all regions of the Northern Hemisphere, while winter extremes are less cold and less frequent. Across North America, average highest summer maximum and minimum temperatures have increased by 1°C since the mid-1960s, while winter extremes have warmed by 3.5°C (5.2.6).
Across North America as a whole, the intensity of daily average precipitation has increased during the past few decades, likely due to a rise in tropical cyclone activity (5.2.6).
A recent rise in hurricane activity in the North Atlantic has been partly offset by a decrease in activity in the northeast Pacific Ocean, resulting in little net global change in frequency. However, there has been a small long term global increase in intense Category 4-5 hurricane events that appears to be directly linked to the rise in global sea surface temperatures. Global climate change may already be a factor, but its signal is as yet lost in the noise of natural variability (5.2.6).
Increases in atmospheric CO2 and changes in climate are affecting global ecosystems.
During the past half-century, average growing seasons for land ecosystems around the world have increased by 1.5 days per decade. In the Northern Hemisphere, this has recently increased to about 3 days per decade, primarily due to an advancement of the onset of spring.
About 50% of recent global leaf area index increase can be attributed to direct CO2 fertilization effects. Over North America, precipitation increases have also been a major factor. However, in boreal regions, the main driver has been warmer temperatures.
While the longer growing seasons have increased annual net primary productivity, the warmer temperatures have also increased ecosystem respiration. This combined response has caused an acceleration of the carbon cycle (5.2.7).
Analysis of global production of six key food crops indicates there has been a small but significant decline in yields of wheat, maize and barley between 1981 and 2002 (5.2.8).
Oxygen depleted waters have expanded and intensified in parts of the ocean as a consequence of increased primary productivity, to the detriment of marine organisms within these regions (5.2.7).
Surveys along the Pacific coast of North America show increased upwelling of abnormally acidic waters, indicative of broader ocean acidification due to rising carbon dioxide concentrations and a general decline in the pH of coastal waters. There has been a corresponding decrease in productivity of local calcareous species relative to non-calcareous ones (5.2.7).
Many changes in climate can now be attributed to human interference with the global climate system.
The pattern of changes in climate around the world over the past half century can be well explained by the combined effects of greenhouse gas and aerosol forcing, but not by natural forcings or variability. Hence, the evidence for a strong human fingerprint in recent global temperature trends remains robust (5.3.1).
Many of the observed changes in the global hydrological cycle, including those related to surface specific humidity, precipitation and river runoff are now at least partly attributable to enhanced greenhouse gas and aerosol forcing. Some regional hydrological changes can also be linked to human influences (5.3.2).
Recent shifts in other climate variables, including increases in regional daily maximum and minimum temperatures, declines in frost days, and increases in degree days, are also increasingly difficult to attribute entirely to natural variability, and therefore likely linked to anthropogenic forcings (5.3.4).
The evidence in support of a human influence on declining Arctic sea ice has been strengthened. A human signal is apparent from the early 1990s onwards and in both winter and summer seasons (5.3.5).
About 90% of the observed global and continental scale changes in physical and biological systems are in the direction projected by climate change impact studies based on model simulations of climate response to greenhouse gas and aerosol forcing (5.3.5).
Impacts of Climate Change Future climates will alter the distribution of water resources over the Earth’s land areas.
On average, increases in evaporative losses due to rising temperatures are expected to exceed increases in rainfall. Hence, average global soil moisture is projected to decrease, with the spatial extent of areas with severe soil moisture deficits and the frequency of short term droughts likely to double over the next century (6.1).
Characteristics of precipitation will also change. Enhanced convective activity is likely to increase the potential severity of thunderstorms and the average intensity of rainfall. While models project that average rainfall intensity is projected to increase by about 2% per degree of warming, heavy rainfall events are likely to be enhanced by about 30% per degree of warming. Comparisons of satellite observations of changes in precipitation intensity with changes in temperature imply that these model projections may be underestimated (6.1).
Ecosystem productivity will be significantly affected by the rise in temperatures, changes in water resources and other factors.
On average, growing seasons around the world are projected to increase by more than one month by 2100. Most Canadian forest ecosystems are projected to become more productive, particularly in northern latitudes and alpine regions, and to shift northward. However, forests are also likely to become more vulnerable to disturbances. The Amazon rainforest could decrease by up to 70% due to reduced regional rainfall and longer dry seasons by the end of the century (6.3.1, 6.3.2, 6.3.4).
Many endemic plants and vertebrate species in key biodiversity hot spots of the world could be at risk of extinction. Those that are currently sparse and have narrow temperature or habitat ranges, or have limited genetic variability, are most at risk (6.3.1).
As the summer season extends and sea ice retreats, marine productivity within the Arctic Ocean is expected to increase, particularly in the early part of the season. Impacts on the food chain will be complex. Species that have been identified as especially vulnerable are narwhals, polar bears and hooded seals. (6.3.3).
Ocean pH could drop about 0.5 units under a high CO2 world, severely impacting organisms that form shells or skeletons of calcium carbonate, since a decline in pH increases the solubility of calcium carbonate (6.3.5).
Ice and snow will continue to retreat, and sea levels to rise.
Climate models show strong agreement in projecting an ice free Arctic Ocean in late summer by 2100. Some suggest this may occur as soon as 2040. These changes in Arctic ice cover appear to be unprecedented within the past million years. The Canadian Archipelago may be one of the last refuges for multi-year ice (6.4.3).
By 2100, the global extent of permafrost may already have been reduced by more than 20%, causing large scale changes in landscape and hydrology. Permafrost decay may result in the release to the atmosphere of significant amounts of old carbon stored in the permafrost layer, as carbon dioxide or methane (6.4.1).
Warmer ocean temperatures could cause significant releases of methane from ocean floor hydrates in shallow waters during the next few centuries. This could cause a significant climate feedback (6.4.1).
The Greenland ice sheet will decline in volume over the next few centuries. It may gradually recover over subsequent millennia provided future global warming remains modest. However, greater warming increases the likelihood of exceeding a critical threshold for eventual total ice sheet collapse (6.4.2).
Total sea level rise from all sources could be between 0.5 to 1.4 m by 2100, much greater than the recent IPCC estimates. Recent trends in temperature and sea level rise appear to support the argument that the IPCC projections are more likely to be underestimates than overestimates (6.4.2).
Many weather extremes are expected to become more severe and frequent.
In general, by mid-century, today’s one in twenty year heat wave will likely occur every 10 years in high latitudes, every 5 years in mid latitudes, and much more often in tropical regions. However, by 2100, the frequency of extreme cold outbreaks in most areas of mid to low latitudes of the Northern Hemisphere is projected to decrease by 50 to 100% (6.5.1).
By 2100, the average return period of extreme precipitation events is expected to be one-third that of today (6.5.1).
While future response of tropical cyclone activity to warmer climates remains uncertain, most experts agree that, everything else being equal, it would be surprising if rising SSTs did not increase their activity, particularly in terms of intensity. However, the total number of tropical cyclones over the Atlantic may actually decrease (6.5.2).
The total number of mid-latitude winter storms is expected to decrease over the next century, but the number of intense storms is projected to increase. Mid-latitude autumn storms are likely to move across the North Atlantic faster, with a storm track somewhat wider and further north than today (6.5.2).
In heavily industrialized regions, the combined effects of increased production of ozone pre-cursors and warmer temperatures are expected to increase the annual frequency of days with high ozone concentrations above acceptable thresholds (6.6.1).