Sachem skipper butterfly

The known impact of climate change

Documented evidence of the biological impact of climate change

How can we be sure that a change in the environment is actually the result of climate change?

Because the Earth’s 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 the climate.

Climate-linked changes have been seen in a wide range of biological systems – from the blooming of spring flowers to the nest building of 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 the distribution of ‘warm’ and ‘cool’ genotypes in the north-east of the USA is changing – genotype frequencies in New York populations in 2002 were close to those seen in Missouri, nearly 1,000 miles (1,600 km) further south, in 1946.

Sachem skipper butterfly

This butterfly (Atalopedes campestris) 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

This Californian bay is a small region where northern species of fish (which are found up to Alaska) and southern species (seen in Mexico) overlap. A comparison of sites surveyed in 1931 and in the 1990s found that numbers of nearly all southern species had increased while those of nearly all northern species had decreased.

Mountain plants

A study of 171 forest plant species at different elevations (0 to 2,600 metres) in western Europe found that the optimum altitude of species had risen on average by 29 metres per decade during the past century.


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.

Lead image:

Sachem skipper butterfly, photographed in the Columbia Bottom Conservation Area near St Louis, Missouri, USA, in 2013.

Andy Reago and Chrissy McClarren/Flickr CC BY


About this resource

This resource was first published in ‘Health and Climate Change’ in January 2009 and reviewed and updated in September 2014.

Statistics and maths, Ecology and environment
Health and Climate Change
Education levels:
16–19, Continuing professional development