The known impact of climate change
How can we be sure that a change in the environment is actually the result of climate change? Because climate cannot be deliberately altered, the best evidence is generally association - when the observed impact corresponds to a period of climate change. This association can be strengthened if:
- other possible causes can be eliminated
- a plausible mechanism exists to explain the link to climate.
Climate-linked changes have been seen in a wide range of biological systems - from blooming of spring flowers to nest building in birds. Some of the best-documented cases use data collected over many years, often not for climate change purposes:
'Drosophila robusta' (fruit fly): Analysis of data going back to 1946 shows that distribution of ‘warm’ and ‘cool’ genotypes in northeasten USA is changing - genotype frequencies in New York populations in 2002 were close to those seen in Missouri, nearly 1000 miles further south, in 1946.
Sachem skipper butterfly: The butterfly has spread from California to Washington State (420 miles) in just 35 years - in one particularly warm year it moved 75 miles northward.
Monterey Bay: The bay is a small region of overlap between northern species of fish (which are found up to Alaska) and southern species (seen in Mexico). A comparison of sites surveyed in 1931 and in the 1990s found that the numbers of nearly all southern species had increased and those of nearly all northern species had decreased.
Mountain plants: A study of 171 forest plant species at different elevations (0 to 2600 metres) in west Europe found that the optimum altitude of species had risen on average by 29 metres per decade during the past century.
Mushrooms: An analysis of 34 500 dated Norwegian herbarium records for 1940-2006 revealed an average delay in fruiting of 12.9 days since 1980.
Studies of sites such as Monterey Bay, where communities overlap, have found that cold-adapted species are always in decline while warm-adapted organisms are always on the increase. Such shifts have been seen in 294 species, right across the globe, in animals as diverse as ocean fish, tropical birds and European butterflies.
All change…
Climate change can be expected to affect certain animals because of their known dependence on climate.
The nine-banded armadillo, for example, requires more than 38 cm annual rainfall and can survive only at latitudes with fewer than 20-24 days below freezing and fewer than nine consecutive days with below-zero temperatures per year.
Tropical insects appear to be at the limits of their thermal tolerance. They may be highly vulnerable to climate change, more so than their temperate relatives.
Climate change may also have an impact on animals, such as many reptiles, where the sex of offspring is dependent on temperature. Below incubation temperatures of 28°C, map turtles produce only males; higher than 30°C and only females are born. Even now, single-sex nests are common.