Comment: Will we eliminate Earth's Ice-Age cycle?
In the last 400,000 years, we have sunk into ice ages every 100,000 years; we’re currently in the latter half of an ice-age cycle — known as an interglacial — with the maximum extent of ice coverage in the preceding glacial period occurring about 22,000 years ago. Two natural factors combine to predominately affect our glacial state: changes in Earth’s tilt, or obliquity, with respect to the sun, and changes in the precession of its axis and its orbit. Gradual changes in Earth’s axis and orbit cause regular changes in the amount of sunlight reaching us, which contributes to the formation or melting of large ice sheets.
On the surface, the 100,000-year ice-age cycle of late corresponds neatly with variations in another orbital parameter: the eccentricity of Earth’s elliptical orbit; however, these changes in eccentricity are far too weak to cause any significant effect on climate. Of course, it’s also likely that other factors within the Earth system amplify the effects of orbital changes, leading to larger impacts on climate. These factors might include sensitivities in the carbon cycle, ocean system or methane balance — all of which are complicated and difficult to predict — that would lead to indirect changes in Earth’s energy balance through the accumulation or removal of atmospheric greenhouse gases.
Interestingly, the 100,000-year ice-age cycle also illustrates that planet Earth can reside in either warm, mostly ice-free states or a cooler state with ice caps of varying sizes, even with the same amount of sunlight reaching the planet. This idea of bistability in Earth’s climate was one of the first questions about planetary habitability to be addressed when the advent of mainframe computers in the 1960s made scientific computing possible. This question continues to be explored in computer climate modeling research today.
As an example of this bistability, consider a modeling scenario where we warm Earth (either by accumulation of greenhouse gases or by somehow brightening the sun). Eventually, we will reach a critical point where the ice caps altogether vanish and a “greenhouse Earth” state many degrees warmer than today sets in. Continuing this thought experiment, if we now cool Earth back down to where we started (either by removing greenhouse gases or by dimming the sun), Earth will not immediately revert to its former glaciated state. The growth of ice caps is much slower than their loss through melting, and only a vast amount of cooling would eventually allow a greenhouse Earth to again fall into a glacial state.
Throughout Earth’s history, the melting and regrowth of major ice sheets has followed the patterns of orbital variations and natural changes in greenhouse gas levels, but human activity may be changing this. Changes in land use and industrialization are becoming a dominant driver of global temperature and atmospheric greenhouse gas levels. Quite possibly, anthropogenic climate change could delay the onset of the next glacial cycle or perhaps even cause the ice-age cycle to altogether cease. Continued aggressive emissions with little mitigation could lead to future increases in carbon dioxide that cause the loss of major ice sheets. Even if we do not lose all of our ice cover right away, we could inhibit the natural replenishment of polar ice due to changes in Earth’s orbit and axis over long periods of time. Thus, anthropogenic climate change could shift Earth toward a greenhouse regime with little hope of recovering major ice cover.
What the loss of the ice-age cycle would mean for our civilization is open to debate. Reduced ice cover might benefit frigid regions like northern Canada and Siberia by providing temperate conditions for industry, more arable farmland and additional space for a growing population. Future generations would also not have to worry about recurring changes in ice coverage, which could potentially allow for more stable and long-lived infrastructure for such civilization.
On the other hand, these potential benefits would come at the expense of people living elsewhere. Some effects of climate change would be especially pronounced in the tropics, where shifting patterns of rainfall and desertification are already contributing to increases in emigration. The continuation of such warming could also render farmland at midlatitudes less productive, thereby placing even more demands upon newly arable polar resources. If Earth were to lose all or most of its polar ice, we would be consigned to a greenhouse future where the melted poles provide some of the planet’s best real estate. And this doesn’t even factor in all the loss of land due to sea-level rise.
The planet has been there before. All ice has melted, sea levels have been much higher, and temperatures and carbon dioxide levels have been higher. But humans weren’t alive then. Human civilization is now an integral part of Earth’s climate system. And our current trajectory, left unabated, could plummet Earth into a virtually inescapable greenhouse future the likes of which humans haven’t seen and for which we are not prepared.
But it is also within our power to steer the planet toward a more viable and resilient future through reductions in our energy consumption and investments in sustainable sources of energy. Although concern over the distant future of the ice caps is certainly not the most prominent driver of policy, our actions today may exert unprecedented influence on our climatic future.