The global commons: Climate brief: Early spring, late winter

The manner in which progressive global warming may affect Earth’s ecosystems in the future is difficult to predict. However, recent studies showing how higher CO2 levels and slightly warmer global temperatures have affected the biosphere so far offer clues about what may lie ahead. Two of these studies have demonstrated that, since the 1960s, warmer average temperatures have brought an earlier spring and a later winter over the higher latitude areas of the Northern Hemisphere, advancing the growing season by about 7 days in the spring and extending it about 2 to 4 days in the fall. This extended growing season, along with elevated levels of CO2, has spurred greater plant growth over a wide swath of territory, including Alaska, Canada, Scandinavia, northern Europe, and northern sections of Russia and China [1] [2] [3].

Previous studies have shown about a 10-percent reduction of snow cover associated with warmer temperatures in higher latitudes, which probably translates to quicker warming of the soil and a faster start to spring growth; the increased CO2 available probably also adds to photosynthesis rates over the growing season. The increase in plant growth – which may be as great as 10 percent in the affected regions – provides some of the best direct evidence so far of a large-scale ecosystem response to climate change. This response is not likely to be universal, however. Higher temperatures and less rainfall in some areas may decrease soil moisture levels and actually suppress growth and agricultural yields [4] [5].

The effects of global warming will not be confined to plants. Field studies of a small North American butterfly, known as Edith’s checkerspot butterfly, have shown the first convincing evidence that the geographic range of an animal species has shifted in response to climate change. Scientists have long predicted that as global temperatures get warmer, the geographic ranges of plant and animal species will shift toward the poles or to higher elevations to maintain their preferred temperature conditions. This is precisely what happened to the checkerspot. Over a number of decades, as global temperature was rising slowly, butterfly colonies on the southern limit of the range failed to survive, while new colonies formed on the northern limit of the range and also at higher elevations. This shift worked out well enough for the checkerspot, but such range migration may not always be possible in the future. Global development and habitat loss will very likely stand in the way of some species, leaving a shrinking range of habitats and increasing the possibility of extinction for many species as temperatures rise [6] [7].

The Greening Affect of a Longer Growing Season

Geographic distribution of the change from 1982 to 1990 in normalized difference vegetation index (NDVI) of land areas north of 27.75 degrees N, expressed as the average over the northern actvie growing season of May through September. NDVI increase in percentage over 9 years, determined by linear regression of year to year northern growing season averaged NDVI, aggregated to 0.25 x 0.25 degree pixels from original 8 km resolution data. Only estimates with a positive slope and a statistically significnat (10% level) regression coefficient were contoured. There were no pixels with statistically significant negative slopes. Again, the emphasis is on the limitations of regression analysis on short term samples. Source: R.Nynemi, C. keeling, G. Asrar and R. Namani, “Increased Plant Growth in the Northern High Latitudes from 1981 to 1991,” Nature, Vol. 386 (1997), p. 698.