Pollen data shows humans reversed natural global cooling | John Abraham

Current temperatures are hotter than at any time in the history of human civilization

In order to understand today’s global warming, we need to understand how Earth’s temperatures varied in the past. How does the rapid warming we see now compare with past natural climate changes? Also, how long have humans been having an impact on the climate? These are some questions that can be answered through paleoclimate studies. Paleoclimate research uses natural measurements of the Earth’s temperature. Clever scientists are able to estimate how warm or cold the Earth was far back in time, way before we had thermometers.

Readers of this column are probably familiar with some of these paleoclimate techniques that may use ice cores or tree rings to infer temperature variations. A different method that uses plant distribution was a technique used in a very recent study published in Nature. That technique used pollen distribution to get an understanding of where plant species thrived in the past. Those distributions gave them insights about the temperatures. On the surface, it’s pretty straightforward. Tropical plants differ in major ways from plants that live in, say, the tundra. In fact, plants that thrive where I live (Northern USA) differ from plants that populate landscapes further south.

Pollen works well as a temperature recorder because plants have specific temperature ranges that they can tolerate. By combining the temperature requirements for dozens of different plants that we can recognize from their pollen, we are able to narrow down the possible temperatures at the location where the pollen was collected.

We use pollen rather than other plant fossils because pollen is widespread each spring and settles to the lake bottom where it is surprisingly resistant to degradation. We wash the samples of lake bottom mud with acids that can dissolve minerals, but the pollen can tolerate it. It lasts up to millions of years with degrading.

The major significance here is temperature across two continents over the last 11,000 years. The paper provides a geologically long-term perspective on recent temperature changes in the Northern Hemisphere and the ability of climate models, such as the National Oceanic Atmospheric Administration and National Center for Atmospheric Research (NCAR) models used in the study, to predict the changes. Climate simulations do a strikingly good job of forecasting the changes.

I would say it is significant that temperatures of the most recent decade exceed the warmest temperatures of our reconstruction by 0.5 degrees Fahrenheit, having few -- if any -- precedents over the last 11,000 years. Additionally, we learned that the climate fluctuates naturally over the last 11,000 years and would have led to cooling today in the absence of human activity.