|How do paleoclimate scientists determine the temperatures of the past? |
The primary source of information about past climates going back many thousands of years is contained in mud that accumulates at the seafloor and in depressions on land. Remains of plants and animals from the past are preserved there. They can be identified and most of them have living counterparts with known climate preferences. So we can translate the biotic assemblages into estimates of temperature and other climate variables. Besides organic remains, the sediment can contain a large variety of geochemical fingerprints of past climate.
With extreme heat gripping parts of the globe this summer, people are wondering when the last time Earth was this hot. What can the research that scientists like you have done tell us about that?
We can provide the long-term context of climate change that’s needed to put ongoing changes in perspective of what’s normal or extreme. From that perspective, recent changes don’t look natural. The Earth has entered a new climate state, one characterized by a global warming level of more than 1 degree Celsius above pre-industrial levels, and we won’t be going back any time soon. The last time Earth experienced a climate with this level of global warming was at least 100,000 years ago.
What do you think is important for people to know about looking at the climate now and comparing it to the past?
The changes that humans have set into motion by increasing the level of heat-trapping gasses in the atmosphere will play out over many centuries if not millennia. Looking at climate changes of the past shows that ice sheets and deep oceans move slowly compared with human lifetimes, but they are major controls on the climate system.