Table of contents key findings

Figure 12. Changes in annual number of cold and warm temperature extremes across Canada during the second half of the 20^th 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 America^623,627.
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 forcing^623,628-630.
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 increase^631-632.
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 variability^634-642.
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 20^th 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 variability^643-666.
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 communities^{651,653,665,667-671}.

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 species^40,672-683.

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 activities^677,679.

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 patterns^429,684-693.

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