Most people have a hard time imagining that earth’s climate hasn’t always been what it is today. And the global warming that we see occurring now is perhaps even more frightening because the human species has grown up in a veritable Eden. But the earth is warming, and the question is “Why?”

I won’t delve into the politicized topic of human pollution, although I myself am of a “better safe than sorry” mindset. However, there is geological and fossil data that suggests warming and cooling occur on earth in regular cycles– cycles which can cause dramatic upheaval to the organisms that inhabit this planet, often leading to massive extinctions, followed by evolutionary explosions of new species during recovery periods.

Scientists have found that changes in marine life diversity, sea levels, and climate tend to co-occur in regular patterns, spanning every 60-140 million years [1]. Every 60-ish million years there tends to be some cataclysmic disaster (think the asteroid and the dinosaurs, for instance). While these disasters are difficult to imagine as occurring cyclically– after all, how can one predict asteroids?– some physicists are addressing the possibility that vulnerability to asteroids may well occur cyclically, increasing in likelihood at certain points along the earth’s astronomical trajectory. (See Dark Matter and the Dinosaurs by Lisa Randall.) However, beside this 60 million year cycle, a more subtle cycle also occurs every 140 million years. And perhaps more interestingly, THAT cycle coincides with major ice ages, one out of which we are currently coming.

Above is a diagram from Ruddiman (2001) I’ve adapted to show the ~140 million year cycles coinciding with lows in temperature (blue line/red asterisks), which are effectively ice ages. You can see at the far right we’re currently coming out of an ice age, and therefore should expect to see a certain amount of warming– I’m assuming up to 10°C over the next several million years if estimates are accurate and past trends hold true.

While research is still in early days, scientists have found that many of these cycles seem to coincide with changes in mantle plume activity, which is an upwelling of hot liquid rock coming from the earth’s mantle. This work suggests that likewise mantle plume activity may occur cyclically, leading to major changes in the earth’s climate and, when most active, to vulcanism and continental breakage and drift.

Regardless of whether human activity has hastened this next warming cycle, most likely the earth will continue to warm to a significant degree. With that, its landscape will change. In addition, oxygen levels could well change dramatically, as recent research suggests has happened cyclically in the past in relation to temperature change and the carbon cycle [2]. One reason mammals may well have remained small during the Jurassic and Cretaceous periods is not solely because we could better hide from dinosaur predators but because mammals do not have as efficient lungs as archosaurian reptiles and their descendants. Big bodies depend on lots of oxygen.

Therefore, it’s possible someday we and most other large mammals on earth may find the air far too thin to breath. I just hope by that point we’ve invented new ways to survive…