Global temperature records go back less than two centuries. But that doesn’t mean we have no idea what the world was doing before we started building thermometers. There are several things—tree rings, isotope ratios, and more—that record past temperatures. Using these temperature proxies, we’ve been able to reconstruct thousands of years of our planet’s climate.
But going back further is difficult. Fewer proxies are preserved over longer periods of time, and samples become rarer. By the time we go back more than a million years, it’s hard to find enough proxies from around the world and over the same time period to reconstruct a global temperature. There are a few exceptions, such as the Paleocene-Eocene Thermal Maximum (PETM), a sudden warming event about 55 million years ago, but few events that old are nearly as well understood.
Now, researchers have used a combination of proxy records and climate models to reconstruct Earth’s climate for the past half billion years, yielding a global record of temperatures that stretches back nearly to the Cambrian explosion of complex life. The record shows that, with one notable exception, carbon dioxide and global temperatures are intimately linked. That’s a bit surprising, given the other changes Earth has undergone during this time.
Climate from the past
The work done here by an international team involves a combination of proxy data and climate models. While there are a number of land-based proxies, these often have very large uncertainties. Therefore, the researchers focused on one type of proxy: the ratio of oxygen isotopes found in the shells of marine organisms. There are some questions about its accuracy, because its use requires that the ratio of these isotopes in the oceans has remained constant over time.
To compensate for this, the researchers used two methods to convert these proxies into temperatures. One method assumed that oxygen isotope ratios in seawater remained constant; the second method used a slow, steady change over the time period.
Climate models provide a way to convert these proxies, which are typically derived from a single geographic location, into a global temperature. By using details such as continental configuration and carbon dioxide levels, the models can estimate what reasonable global temperatures are consistent with the proxy data, meaning a specific temperature at a specific location on the globe. The researchers used an ensemble of climate models so that the results were not dependent on a specific implementation of atmospheric physics.
The results, which the researchers call PhanDA, estimate global temperatures over the past 485 million years, going back to the end of the Cambrian, the period when the major groups of modern animal life diversified.
So what does PhanDA look like? One important feature is that it overlaps with the Cenozoic, which began with the mass extinction that wiped out all non-avian dinosaur lines. We have a better history of Cenozoic climates, so these provide an important test of whether PhanDA temperatures match those obtained independently. The consistency between them is an important validation of the new work.
Overall, the researchers conclude that global average temperatures have likely ranged from a low of about 11°C, seen during recent ice ages, to a high of 36°C, seen about 90 million years ago, although similar extremes were seen during the PETM. Other major climate events, such as the warming caused in the aftermath of the eruptions that formed the Siberian Traps, were noted in the report. There have been both long periods of warming trends (such as one that spanned most of the Mesozoic) alternating with cooling trends (which have dominated the current Cenozoic). The researchers suggest that these are caused by the assembly and breakup of supercontinents.
A greater portion of this period was spent in warm greenhouse climates (41 percent of the period) than in icehouse climates (31 percent). The researchers found that most of the difference between these climates occurs in the polar regions. Changes do occur in the tropics, but they are considerably smaller in magnitude. Thus, during an icehouse period, the difference between equatorial regions and high latitudes is on the order of 30° to 50° C. By comparison, during greenhouse periods, the difference from equator to pole is on the order of 15° to 25° C.
