In 2008, I and my colleagues published a study which appeared to confirm that current global warming could cause large losses of species and a loss of biodiversity. Four years later and a new approach to the data has changed our ideas on the effect of global warming on species. Our new study suggests that biodiversity actually increases in times of warmth - though this doesn’t provide much comfort in terms of current climate change.
The new research
Some of our previous findings are upheld in the new study. Origination and extinction are still more frequent in greenhouse periods. But biodiversity now increases rather than reduces with warmer temperatures. Furthermore, the effect of temperature still holds even after accounting for numerous other environmental factors which are thought to influence biodiversity.
A subtle difference is in the timing of events. The new study shows that extinction happens after biodiversity rises. This suggests that extinction may be caused by competition rather than being caused directly by temperature.
A warmer Earth may promote biodiversity in the very long term, but this probably depends on the origination of new forms of life, which we know generally takes thousands or millions of years. Hence, the new study doesn’t provide much comfort in the face of current climate change. In contrast, the majority of work on current ecosystems suggests that climate change will cause global loss of diversity, though it may increase locally in some places.
These new results do conform much better to the geographic pattern of life on Earth, which suggests that warm climates are good for biodiversity in the long term.
Learning from the past
In 2008, we used the most comprehensive fossil data then available, going back 540 million years. We compared diversity in these fossils as Earth’s climate fluctuated slowly from icehouse states to warmer greenhouse states. We found that warmer periods experienced greater extinction (including four of the big five mass extinctions) and reduced total diversity. After these extinctions there was a peak in the origination of new species.
Although our first study involved very long timescales, extinction can happen rapidly. And concerned citizens of the planet probably don’t want to risk environmental destruction on the scale of the end-Permian extinction. Many scientists readily accepted our findings, but the response from those studying the fossil record was mixed.
Three general objections were raised to the study. The findings seemed to contradict the spatial pattern of life on Earth, where there is greater biodiversity in warmer regions, rather than less. Second, we focused on the effects of temperature to the exclusion of other possible causes. Third, we used fossil data that contained potential biases.
After addressing these concerns, our new results will probably please those who were more sceptical of the 2008 work.
The use of new data was very important. Up until very recently, people examined broad trends in the fossil record by looking for the first and last fossil of each group they were interested in and assumed that the group was present at all times in between. There is some sense to this approach; it analyses what you know was really there; and the method definitely correctly identifies some important biological signals, such as the times of mass extinction.
However, the method is prone to bias. Some geological periods are better studied than others, so first and last fossils of different groups tend to concentrate in those regardless of the true underlying diversity pattern. In the last decade, a large effort by the scientific community has led to techniques to scrub away that bias. The key is to measure the fossil record much like an ecologist would sample a rainforest; by using a standardised sample, in this case one that is equally fair to the different time periods.
Preparing to change direction
As a scientist, these studies have taken me down a twisting intellectual path. The duty of a scientist, and I believe any responsible citizen, is to let your conclusions be guided by the evidence. But science develops by getting new and better data, hence we always need to be prepared to switch direction when the data tell us to. Although I have always known this in theory, I have never experienced it quite so prominently in my own work. It’s been a big test of how to conduct myself as a scientist. And I believe there is more to come.
There still isn’t much of a scientific consensus over the environmental drivers of diversity through time, and I’m sure this current study won’t be the last word on the subject. Still, it is very humbling to use data that contain the cumulative message of hundreds of fossil studies. Scientists often talk about “standing on the shoulders of giants”. Scientists also stand on the shoulders of thousands of dedicated workers who never achieve fame, but who accumulate the majority of what we know.