Antarctic Ice sheets. Changes in the behaviour of the Antarctic ice sheets have, on average, been more modest that those for Greenland. Past altimeter studies had indicated that, on average, mass gains from increasing snow accumulation over East Antarctica exceeded net ice mass losses from West Antarctica for the 1992-2003 period, thus resulting a net increase in total Antarctic ice mass. However, recent, more advanced analyses, updated to 2006, now indicate that the total ice mass in Antarctica has been shrinking at an increasing rate since 1996. Satellite-based gravity measurements indicate net total ice loss has increased to about 150 km3/year between 2002 and 2005, with the loss from the West Antarctic sheet estimated to be about three times that from the much larger East Antarctic sheet. Total ice discharge from the major glaciers of the West Antarctic region has increased significantly over the past decade or so. Melting along the Antarctic ice sheet margins has also increased and, in most regions, has moved further inland. This increase in melting has not been offset by an equivalent increase in interior snow fall, resulting in a net loss of Antarctic ice volume since 1992. Some researchers, using process studies as well as paleo records for the past deglaciation, suggest the ice volume decline may be linked to rising sea levels and warming ocean waters, since these melt the bottoms of floating glacier tongues and ice shelves that buttress the glacial outflow. However, others note that sediments at the bottoms of ice shelves at their grounding line reduce the effects of sea level rise and that accelerated break-up is more likely temperature related475,531,552-559.
Arctic ice shelves and other glaciers.Progressive ablation during years of warming, culminating in the very warm summer of 2005, caused one of six Ellesmere Island ice shelves to lose 7.5% of its area due to calving of a large ice island. Studies suggest that more such calving may soon follow560.
Most other ice caps and glaciers around the world are also in decline. In northern Canada, the ice caps on Baffin Island are melting at unprecedented rates and will likely have completely disappeared by 2070. The Devon Island ice cap is also decreasing in volume. Almost all glaciers in diverse areas such as British Columbia, Alaska, South America, Europe, New Zealand, Kazakstan, and the Himalayas are in retreat and or thinning. 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%. While melting due to rising temperatures is a key factor, reduced snow fall related to atmospheric circulation changes is also implicated in the decline of some glaciers, particularly on Mt. Kilimanjaro in Africa and in the Himalayas. In some polar regions, such as that of Devon Island, changes in snow accumulation on glaciers and ice caps are also associated with upwind variations in sea ice cover and hence local moisture fluxes561-571.
Other changes in the Global Cryosphere. Spring snow cover extent across the Northern hemisphere has decreased over the past 35 years by -1.28 million km2. The onset of snow melt is now also much earlier, at least partly due to the increased influx over land of dry, moderate air masses that transport with them more sensible heat and less latent heat. The magnitude of these changes amplifies poleward, consistent with a strong positive albedo feedback effect. Changes in winter and fall seasons have been much less significant572-573.
Numerous studies indicate that permafrost temperatures have generally increased over much of the Northern Hemisphere in recent decades, although rates of warming vary significantly from region to region. In some regions, such as Alaska and the Canadian high Arctic, this warming may be more due to increased thickness of winter snow cover, which insulates the ground from heat loss, than to rising air temperatures. Many other observed changes in ground ice appear to be consistent with this ground ice warming. For example, ice wedge development in the boreal regions of the eastern MacKenzie delta has ceased during the past century. The active layer thickness layer has increased in various regions around the circumpolar Arctic and sub-Arctic, and there has been significant loss of thin permafrost. Much of this thaw occurred during the 1920s and since the 1950s. The extent of thaw in Canada appears to be greatest in western regions, where increases in winter temperatures have been large. Models suggest total permafrost area across Canada may have declined by 5.4% since the mid-19th century, and that the active layer has increased by about three-quarters of a meter (although some experts caution that such models tend to overestimate the extent of thaw). Photographic data indicate that permafrost melt has also increased the number of thermokarst ponds, and caused thaw slumps. In western Canada, the number of thaw slumps appears to have increased by 15% between 1950 and the 1970s, and by a much larger 36% since. Local influences, including soil type and ground cover, are important factors. These slump areas become important ecological islands within the landscape, and are gradually contributing to a change in local soil chemistry and hydrology. 240,418,528,574-596.
Annual duration of unfrozen soil conditions in Russia have, on average, also increased by 5 days. However, although average surface air temperatures across northern Eurasia have increased by 1.4°C since 1956, and seasonally frozen ground regions have been warming, there appears to be little significant change in freeze/thaw indexes across the permafrost region of Russia in recent decades. Largest increases in these indexes have occurred in spring in southern Siberia and in northeast Asia. Over Canadian and Alaskan permafrost regions, there has been a significant decrease in the cold season freezing index, while the thawing index during the warm season is also rising in eastern Canada and coastal regions578,597.
Sea Level Rise.Tide gauge studies spanning the past three centuries suggest the rate of sea level rise during that period has accelerated. The estimated 17-20 cm rise in the 20th century is about three times that of the preceding century. Observed rates are higher than those suggested by many models. Average acceleration in sea level rise since 1870 has been about 0.013 mm/yr2. Although there has been a large rise in recent decades, there is no strong evidence that the rate of acceleration in the rise has significantly increased598-603.
There is some disagreement between various studies on the relative magnitude of the various contributing factors causing sea level rise. It appears that, over the past century, melting of ice sheets and glaciers has been a more significant factor than thermal expansion of ocean waters. However, analyses of ocean data sets suggest that thermal expansion may be of increasing significance. Estimates suggest this may have contributed an average 0.92 mm/year over the past four decades, increasing to twice that rate during the past decade. Further complicating such attribution studies is the possible offset of as much as 3 cm from reduced river outflows due to the human construction of land water reservoirs. Some experts suggest that, since model simulations have underestimated recent trends in both ice sheet dynamics and sea level rise, future projections by these same models may also be much too low601,603,604-608.
Circulation and Variability
Satellite records indicate that the height of the global tropopause has increased by 4-7 meters/year since 2001. Much of this rise may be attributable to oscillatory behavior, but some appears linked to rising tropospheric and declining stratospheric temperatures. Global jet streams in both hemispheres have also risen in altitude during past few decades, and shifted poleward. They have weakened somewhat in the Northern Hemisphere and the sub-tropics of the Southern Hemisphere, but strengthened in southern high latitudes. These changes are linked to a significant widening of the tropical circulation belt, increased droughts at the belt margins, and a poleward movement of storms tracks. Over the Pacific region of the tropics, the Walker circulation system that causes subsidence in the eastern Pacific and upward convection in the west has weakened. While models do not capture some of these changes well, they are generally consistent with projections under enhanced CO2 forcing and thus likely at least partly due to human influences. On the other hand, there are also indications that the recent changes in NAO behavior may be entirely due to natural causes305,318,609-612.
Ocean sediments from the tropical Atlantic indicate that at least some of the recent regional changes in Atlantic ocean upwelling,and local climates, may be unprecedented within the past several millennia. One study suggests that the North Atlantic overturning rate has slowed down by as much as 30% over the past half-century. However, investigators acknowledge that this conclusion is based on a sparse set of data. Furthermore, recent observations indicate that the rate of deep water formation in its northern regions that drives this overturning has recovered. Therefore, there is as yet little evidence of a long term weakening of the overturning rate. Experts indicate that better monitoring is needed. Projecting these changes in the thermohaline circulation system is also a challenge, since ocean circulation patterns involve complex relationships with freshwater influx due to ice melt and regional changes in precipitation, surface heating and cooling, as well as oscillatory factors. Thus, given the concerns about data quality and model performances, the response of the thermohaline circulation system to global forcing as projected by models may not be detectable for a few more decades613-616.
Wave heights over northeast Atlantic have increased over the past 25 years. This appears to be caused by intensification of the westerly atmospheric circulation due to changes in the NAO, not due to increased storminess617.
Analysis of the past half century of ENSO indices suggest no significant change in either their variability or persistence over time, although there was some evidence of a more El-Niño like behavior in the Southern Oscillation Index during summer seasons618.
5.2.6 Climate Extremes Temperature. 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. In western Europe, the average length of summer heat waves has doubled since 1880, and the frequency of hot days exceeding the 95th percentile has tripled. Most of the increase has occurred during the past half century. Warm extremes have also become more frequent over the North Atlantic, although not more intense. Canadian studies also indicate a significant decrease in frost days. Some regions, such as southern Quebec, have experienced an increase in the number of thaw/freeze days. However, in south central Ontario, there is little indication of a significant change in freeze thaw/activity. While the NAO and other oscillations are major factors in these changes in temperature extremes on decadal time scales, the longer term trends are consistent with climate models projections of climate response to increased radiative forcing caused by rising atmospheric concentrations of greenhouse gases and aerosols619-626.
a) Cold days
c) Cold nights
b) Warm days
d) Warm nights
Figure 12. Changes in annual number of cold and warm temperature extremes across Canada during the second half of the 20th century. Extremes are defined as the lower and upper 10% of values, respectively, within the distribution of daily mimimum and maximum temperature events. (Vincent and Mekis, 2006, ref. # 625) Precipitation.Across North America as a whole, the intensity of daily average precipitation has increased during the past few decades, and the maximum annual one day and five day rain events have, on average, increased. There has also been an increase in extreme rainfall events over the North Atlantic and western North Pacific. Much of these increases can be attributed to a recent rise in tropical cyclone activity. Extreme precipitation events also increased over much of South America623,627.
In contrast, over Canadian regions of the continent, both average daily rain intensity and the number of consecutive dry days decreased. Thus, the rise in total precipitation in Canada has occurred primarily because of more frequent small to moderate rain events. However, there has been an increase in the number of days with heavy rain events. The frequency of days with freezing precipitation decreased during fall, winter and spring seasons over much of southern Canada, but increased during spring and fall seasons in higher latitudes. Total snowfall decreased in the south and increased in the north. Other changes in related climate variables include decreased blowing snow, increased sunshine in spring and summer seasons but decreases in fall seasons, and more hail storm events in Ontario. As with temperature, experts indicate that much of the regional and decadal variations in these precipitation events have strong links to the NAO, ENSO and other climate oscillatory patterns. However, longer term trends are generally consistent with projected climate response to enhanced radiative forcing623,628-630.
There are also indications that storm activity has increased in high latitudes of the North Atlantic and decreased in mid-latitudes, likely due to a poleward shift in winter storm tracks. On the other hand, storms have become more frequent in the mid-latitudes of the North Pacific, as well as in the high latitudes of the Southern Hemisphere. However, while the number of these winter storms appears to indeed be increasing, past studies appear to have significantly over-estimated this increase631-632.
In the tropics, deep convective storm activity has recently been increasing by 6%/decade, or at the rate of about 45% per degree of regional warming. This also appears to be an important factor in the rise in extreme tropical precipitation events633.
Hurricanes.Over the past two decades, there has been little change in the total annual global number of hurricanes. That is largely because the recent rise in hurricane activity in the North Atlantic has been partly offset by a decrease in activity in the northeast Pacific Ocean. However, recent studies suggest that 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. The tropical storm wave power index along the American Atlantic coast has also increased, largely due to more frequent storms. While some experts have challenged these conclusions, the disagreement appears to be at least partly due to confusion between long term change and shorter term variability634-642.
There is little disagreement amongst researchers about the recent increase in the frequency and intensity of tropical cyclones in the North Atlantic, relative to the preceding decades. However, there is considerable controversy over how significant these changes are within the longer term record of the past century and beyond, and what may have caused the recent rise in activity. Two key factors include sea surface temperatures and vertical wind shear. A rise in the former increases the potential for cyclogenesis, while increased wind shear decreases it. Changes in these two variables can explain most of the variability in Atlantic hurricane activity over the past 40 years, but the relative importance of these two factors in explaining past trends in tropical cyclone activity is still under debate. Some argue that the recent increase in hurricane activity is due to the combined effect of reduced wind shear linked to ENSO behavior and natural variations in low latitude sea surface temperatures linked to the Atlantic Multi-decadal Oscillation (AMO) or other longer term ocean circulation cycles. Various researchers also note that lack of observational tools prior to the onset of regular airborne and satellite monitoring may have resulted in a large under-representation of tropical cyclones prior to the 1960s, leading to a strong bias in the tropical storm record. The recent rise in activity may simply be an intense phase of a long term, natural cycle. However, other experts point at the strong linkage between tropical storm activity and low latitude sea surface temperatures, which have shown a significant rise over the past century that is attributable to human induced changes in radiative forcing (see section 5.3). Furthermore, they suggest that the extent of under-reporting of tropical Atlantic storms in the early part of the 20th century may have been exaggerated because of faulty studies using paleo and proxy data. They argue that, in addition to the interdecadal variability in behavior observed in the data, there has been a long term trend towards increased hurricane activity. Atlantic hurricane seasons also appear to have become longer. In summary, these studies suggest that global climate change may already be a factor in recent hurricane activity in the North Atlantic, but that the global change signal is as yet lost in the noise of natural variability643-666.
The substantial disagreement between experts on trends in tropical cyclone behavior and the attribution of these trends to causal factors underscores the inadequate understanding of the processes involved and of the complexity in the relationship between cyclone behavior and the global and regional climate systems. The recent rise in North Atlantic hurricane activity, for example, may be less due to the absolute rise in regional SSTs than to the changes in the differences between the tropical Atlantic Ocean SSTs and those for the Indian and tropical Pacific Oceans. Wind shear also appears to be linked to these changes in relative SSTs. SSTs in these regions can be affected by natural oscillations, but also appear to have increased over the past century, at differing regional rates, in response to rising greenhouse concentrations and changes in aerosol concentrations. Furthermore, since the development of cyclones into intense hurricanes may involve critical thresholds, these relationships may not be linear. Paleo studies can also contribute to better understanding of the processes involved, particularly on longer time scales. This calls for closer collaboration between the climate system and tropical storm research communities651,653,665,667-671.
5.2.7 Ecosystems Land 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 by between 2.3 and 2.8 days per decade. In Europe, where more than three quarters of plants monitored had similar rates of advancement in their spring awakening, senescence in the fall was delayed by only 1 day/decade. Most of this change in seasons has occurred at latitudes north of 40°N. In some regions, such as northern coastal Greenland, the onset of spring activity has advanced by as much as 14.5 days per decade. While the longer growing seasons have increased annual net primary productivity (NPP), the warmer temperatures have also increased ecosystem respiration. The consequence has been an acceleration of the carbon cycle, with little net change in net ecosystem productivity (NEP). The longer growing seasons have contributed to large shifts in the behavior of ecological species, particularly in colder climates. For example, in the central and eastern United States, birds have migrated northward by 2.35 km/yr over a recent 26 year period, while there is no evidence of a similar southward shift. In some parts of the Arctic, polar bears appear to be shifting their diets to adapt to changing ice conditions. In western Hudson Bay, for example, their consumption of bearded seals has declined, while that of ringed seals has increased. A new species of blow fly has surfaced in Ontario during fall seasons. Over the past century, forest plant species in alpine regions have moved upslope by an average of about 29 meters/decade. Northern ecosystems have also experienced a shift to more woody species40,672-683.
The causes of these ecological shifts are not all temperature related, and can vary with latitude. For example, dynamic vegetation models suggest that about 50% of the global leaf area index increase reported for recent decades 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. Other factors that might affect the regional patterns include ozone exposure and nitrogen fertilization. Therefore, all of these factors, and most of the observed ecological trends, are linked to global scale human activities677,679.
Figure 13. Time series of 5-yr mean anomalies in growing season length index, relative to the 1950–99 mean, for North America. The data are from the observations (thick black line) and various HadCM3 experiments using radiative different forcings. The red line represents results for all forcings, the thin black line for total anthropogenic contributions, the green line greenhouse gas forcing only, and the blue line natural factors only (Christidis et al., 2007, ref. # 672).
To date, many of the observed ecological shifts noted above have been relatively benign, partly because of the ability of species to adapt through migration. However, there are also some worrisome negative impacts on species that may be harbingers of future impact of climate change. In the American tropics, for example, about two-thirds of amphibian species have vanished over the past 20 years. A possible cause may be a destructive fungus that has flourished under warmer nights and increased daytime cloud cover, although other factors may also be involved. Multi-year warm periods also appear to have caused amphibian dieback in Australia. Some polar and mountain species are also becoming increasingly restricted. For example, there is new evidence that longer ice free seasons in the Beaufort Sea and Hudson Bay/Baffin regions is stressing polar bear populations in these regions, causing increased fasting, cannibalism and problematic interaction with Inuit communities. In the high Arctic, recent summer desiccation of ponds that have been present for millennia has changed water chemistry and had profound impacts on ecosystems within them. These cases demonstrate that some species are particularly vulnerable to the impacts of change, either because of their low tolerance to change in climate conditions or because of the potential extinction of their habitats with little option for migration. Within the Eurasian boreal forests, fire risk indexes show significant increases in far eastern regions, where most fires have occurred, although these have decreased west of the Ural mountains. For boreal forests of Canada and Alaska, there has been a doubling between the 1960s/70s and the 1980s-90s of annual area burned, and more than a doubling in the number of large fires. Much of the increase occurred in early and late seasons, and can be linked to changing temperature and precipitation patterns429,684-693.