Ecosystem Responses to Climate Warming

Vegetation response to climate warming in high latitudes has typically been viewed as one in which the vegetation growth, or greenness, increases under the influence of increased CO₂ concentrations in the atmosphere and associated extension of the growing season. In this view, the photosynthetic gains conferred by the modified climate offset respiratory losses associated with higher nighttime temperatures. Indeed, satellite observations of high latitude boreal forests indicated that this was the case – vegetation “greening” occurred between 1982 and 1999. A recent analysis we conducted suggests, however, that this trend has not continued uniformly in time or space. Rather we have found that while tundra vegetation experienced an increase in both peak photosynthesis and growing season length between 1982 and 2003, many forested areas actually experienced a decline in photosynthetic activity and showed no systematic change in growing season length. Climatic warming occurred across the entire region, but the change in the forest response indicates that long-term changes may not be predictable from initial, short-term observations. Fire disturbance has also increased with the warming (see related section on this topic) but does not explain the decline in forest photosynthetic activity.

Satellite-derived map of photosynthetic trends across Canada and Alaska from 1982 through 2003. The colors correspond to the magnitude of the trend over time. The average of growing season values for all areas identified as strongly negative, negative, positive, and strongly positive are shown in the upper right (colors correspond to those in the map). The total area burned annually across Canada and Alaska from 1980 to 2005 is shown in the bottom left.

This is some of the first evidence that high latitude forests may be in decline following an initial growth spurt associated with warming. The reasons for this decline are not certain, but related work points to increased drying as a likely cause. The observed warming and drying are consistent with climate model predictions for the region. These trends have implications for the direction of feedbacks to the climate system. For example, the scientific community long thought that increased greening would provide a negative feedback on CO₂ concentrations, reducing CO₂ via increased photosynthesis, which would moderate climate warming. Our results, which are detailed further in other sections of these web pages, suggest that this may not be the case in boreal North America. Rather, forests appear to be in decline. These results emphasize the importance of longer-term synoptic observations of arctic and boreal biomes. The results are important because they describe how large areas of forest are changing and how those changes are related to climate. The results are increasingly supported by a variety of field studies from other researchers that show systematic changes in vegetation in response to climate variability.

Growing season (June, July and August) photosynthetic activity from tundra and forest areas compared to temperature observations. The satellite data extend from 1982 to 2003 and the temperature data from 1982 through 2000. The dotted lines are statistical fits to the data. Note the close correspondence of increasing temperature and photosynthetic activity in tundra areas, and how they diverge in forest areas.

©Woods Hole Research Center, 2007