A Second Warming: Temperatures Then and Now

by Ray Grigg

Paleoclimatologists have noticed that the sudden warming taking place on Earth today — almost certainly caused by human greenhouse gas emissions — closely resembles the warming during the Paleocene-Eocene Thermal Maximum (PETM) that began about 56 million years ago. Scientific analysis of this 200,000 year epoch is particularly useful if we are to understand the consequences of our individual and collective decisions to continue burning huge quantities of fossil fuels.

Geological records provide a fairly clear record that the radical change in Earth’s climate marking the beginning of PETM was probably the result of a sudden and massive surge in greenhouse gases. This surge was caused by extensive volcanic activity as the large super-continent, Pangea, began breaking apart. As magma ignited carbon-rich sediments, about 2 billion tonnes per year of carbon dioxide were released into the atmosphere. When the resultant warming reached about 2°C, frozen methane began to melt from hydrates stored deep in the oceans and from permafrost locked in polar latitudes. Global temperatures quickly rose by as much as 9°C.

But “quickly” distinguishes our present situation from the PETM. During this ancient epoch, temperatures rose to 5°C over a period of 10,000 years, and another 10,000 years passed before the 9°C was reached. Relatively little extinction occurred on land because its plants and animals had time to either adjust or move northward to more comfortable temperatures — an available option since the continent of Pangea was still mostly intact. Consequently, palms and tropical forests covered high latitudes, while equatorial fish and sea turtles swam in northern waters. Mammals slowly got smaller to better cool themselves in the higher temperatures. The largest extinctions occurred in the oceans where acidification dissolved the calcium shells needed by some marine animals. Generally, however, the planet’s ecosystems adapted fairly well to the changing conditions.

The similarities between the PETM and the present are remarkably close — except for the time scale. The temperature and acidification changes that occurred over 10,000 years are now occurring in 300 years. The PETM’s 2 billion tonnes per year of carbon dioxide emissions are dwarfed by the 35 billion tonnes per year we emit. Distinct continents and our massive human settlements are obstructions that prevent many plant and animal species from migrating to more favourable locations. The current extinction of both flora and fauna is literally equivalent to a crash because circumstances are changing too quickly to allow for species adaptation.

If we can notice such ecological changes from the perspective of a single human lifetime, then in geological terms the transformations are virtually instantaneous. Such noticeable changes simply dwarf normal variability and, defined by scientific measures, the magnitude and seriousness of our disruptive influence on Earth’s biosphere is catastrophic.

Despite the obvious, however, we are not inclined to notice. As a human species, we are biologically tuned to live in the present. Our sense of time and change is biased toward the immediacy of moments — even years and decades of accurate memory are compromised by variables that are extremely subjective. Perhaps our best sense of very long expanses of history is a developed skill acquired by academic study and nourished by an exercise of the imagination. We are not psychologically designed to appreciate how the collateral damage of our present actions may be detrimental to our future interests. Without scientific knowledge, for example, we would never notice the similarities between our present ecological situation and 56 million years ago.

It is this comparison that makes the 2°C threshold of global temperature increase during the PETM so important. This was the approximate temperature at which carbon dioxide emissions triggered huge releases of methane. Modern science knows that methane takes about 10 years to break down into carbon dioxide, and is 20 to 25 times more effective as a greenhouse gas during that period. And we know that we have the option of controlling carbon dioxide emissions. But, should methane hydrates begin to melt from warming oceans, or should other methane be released from thawing permafrost, then we slide into a phase of uncontrollable global warming. We have already increased global temperatures by 0.8°C above normal levels, and our existing emissions commit us to another 0.3°C (NewScientist, Mar. 2/13). With no viable emission controls in force and greenhouse gases continuing to rise, we are gambling that 1.1°C is a safe risk, and that even higher temperatures should not be worrisome.

Global temperature averages, however, misrepresent the security of our actual situation. The world’s oceans can absorb huge quantities of heat so they are deceptively reassuring, while their churning currents complicate our ability to predict the brooding effect of a hotter atmosphere. And the poles, we know, are warming nearly four times faster than the global average — record melting of Arctic Sea ice and rapid thawing of permafrost in northern Russia and Canada attest to this trend. In other words, we may have already exceeded one of the safety margins for preventing uncontrollable methane emissions.

Accurately measuring and understanding a 20,000 year process that is compressed into a mere three centuries is an extremely difficult challenge. Expect some confusion and errors. But paleoclimatologists know the beginning of the story and the possible ending. It’s the detail in the middle that’s the unfolding mystery, and the ending we decide to write.