Since life first evolved on our planet some 3.7 billion years ago, the Earth’s climate has been constantly changing. About 90 million years ago the climate was much warmer than today. There was no ice on the planet and there were even tropical rain forests in the polar regions, near what is now modern day Australia. There have also been intensely cold periods and throughout the Earth’s history, there have been devastating ice ages, the last one ending about 12,000 years ago when the polar ice caps covered much of Britain and Northern Europe.
Climate change is very much a hot topic for today. The current rate of change seems unprecedented and the world is heating up by about 0.2 degrees C per decade. By the end of this century the global temperature, predicted by various computer models, is estimated to be about 1.5 to 2 degrees higher than today. Our planet’s weather is a complex system influenced by many interconnecting natural factors such as fluctuations in ocean currents, volcanic eruptions, and even solar activity. Human factors include greenhouse gases emissions such as carbon dioxide, release of pollutants into the environment and activities such as deforestation and intensive farming. Natural and man-made factors both play a part and we are still learning how the extremely complex system of Earth’s weather works.
According to the UK Meteorological Office the last decade ending in 2020 was the second hottest in the UK on record. Want to guess at which decade was the hottest? It was the one before - from 2001 to 2010. Climate change is not merely a matter of rising temperatures. Changes in rainfall and increasingly frequent extreme weather events such as severe storms and unseasonably high or low temperatures are also brought about by climate change. So how are these changes affecting British butterflies?
Butterflies have been around a very long time, far longer than humans. Some fossil records dating from 200 million years ago, found in Germany, contain the ghostly images of the first butterflies. The insects have been evolving since those ancient times. Because our current rate of climate change is so high, the process of species evolution cannot keep pace and many varieties of plants, animals and insects are threatened with extinction as a consequence. One of the results of climate change is that our winters in Britain are becoming milder and summers hotter and wetter. A natural assumption would be that butterflies should thrive in these warmer conditions. After all butterflies enjoy sunshine and are very much a feature of our summers, when the hours of daylight are longer and the temperature is higher.
The picture is not as straightforward as would seem from first sight as any of our British butterflies overwinter as dormant caterpillars. While the winter temperatures are low the caterpillars remain in suspended animation. Mild winters are causing caterpillars to stir from their dormancy and to search fruitlessly for their particular foodplant, which of course is absent in the winter months. This unwanted activity results in the caterpillars using some of their vital energy reserves needed to survive the winter, and so they perish before the spring arrives. They also make themselves more vulnerable to predators such as birds as they roam from their secluded, and often camouflaged, resting places in search of food. Our warmer British winters are causing an increase in the casualty rate of caterpillars and fewer survive to the following spring.
A positive effect of mild winters is that butterflies are taking to the wing earlier in the year, and in some species this has enabled them to have two broods a year rather than just one. The Dingy Skipper is one such example and second broods of this butterfly have become more common over recent years.
According to the charity Butterfly Conservation, climate change is causing shifts in the distribution of butterflies in Britain. Southern species of butterfly have been expanding northwards over the last 50 years. Not all species have responded equally, and sometimes lack of a suitable habitat will limit the range to which some butterflies can spread.
Extreme weather events have always been a feature of British weather but are becoming more frequent. Severe summer storms are becoming more common and in some cases their impact can be devastating. Last year in 2020 we had high winds and stormy conditions in late June. This coincided with the flight period of the relatively rare Purple Emperor butterfly. This insect roosts overnight in trees, particularly the males who find shelter relatively high in the woodland canopy. It is thought the stormy conditions over just two days killed many butterflies while they roosted. The result was that the Purple Emperor had one of its poorest years in Buckinghamshire, Berkshire and Oxfordshire. Another challenge for butterflies are prolonged dry spells, which have been a feature of our weather over the last few years. Drought conditions can wipe out butterfly caterpillar food plants and also reduce the flowering period of plants, thus depleting supplies of nectar for the adult insect.
Climate change is a very complex area and its progress and impact can be unpredictable. There is a major responsibility on all governments and individuals to do what we can to drastically reduce our impact on the climate. Whether we are doing enough and taking action sufficiently promptly remains to be seen.