On the scale of a human lifetime the Earth’s climate appears to be relatively stable and predictable: we plant in the spring, we harvest in the fall, and if we are fortunate, we might vacation in the winter someplace warm. We understand that Earth’s climate does change over time, but think that these changes will be noticeable only over many generations. The Earth has warmed, on average, by about 0.8 degrees Celsius in my lifetime (1968 – present) ; however, I would not have known this had the work of weather and climate researchers not been published and publicized for me to see.
However, there have been many media references  recently to climate “tipping points”, which are described as rapid and large-scale shifts in the climate from one stable state to another stable state. They are also depicted as points of no return, at which Earth’s climate may become unstable and undergo a runaway shift to a state that is potentially inhospitable to life as we know it. Is this all so much hype, or could we face a catastrophic tipping point in the relatively near future?
Stability, the Snowball Earth, and the Ice-free Earth
The Earth’s climate is currently in a state of relative equilibrium where there is permanent ice at the poles and no permanent ice near the Equator.
After passing a tipping point, we would likely careen towards one of two other equilibria: an ice-free Earth or an ice-covered Earth.
“Snowball Earth” theories about a time in Earth’s distant past when the planet was completely covered in ice have been circulating since the early to mid-twentieth century. The theories originated from geological observations pointing towards the onetime existence of long-standing glacial ice near the Earth’s Equator . Soon after that discovery, two climatologists independently created climate models supporting the idea that Earth’s climate could be stable when the planet was covered in ice .
The models, called “energy balance climate models”, were based on the idea of a feedback loop. As the Earth becomes covered with more ice at lower latitudes, more sunlight (solar energy) is reflected away from the planet. Less energy is absorbed by Earth’s climate system, so temperatures cool further, more ice forms, and the cycle intensifies. This is an example of a runaway cooling effect. We are not concerned about this effect today, but the idea of feedback loops within Earth’s climate system is important.
As climate science progressed, additional geological studies further supported the “snowball Earth” theory. Finally, in 2002, the original energy-balance model was rigorously tested  (Figure 1).
Figure 1 – Latitude where permanent ice stopped forming as a function of sunlight reaching the surface (Es), or amount of CO2 in the atmosphere. At the present day, Es = 1.0. Graph reprinted from .
The chart above demonstrates that, with the current solar energy contribution to Earth, there are three possible equilibrium points in the Earth’s climate. First, the Earth of today, with ice near the poles and none at lower latitudes. Second, an ice-covered snowball Earth. Third, an Earth with no permanent ice at any latitude. If we start at today’s climate and trace the chart, we see that once the permanent ice near the poles recedes past 65 degrees, Earth moves from our current condition to an ice-free condition very quickly (on a geologic time scale). Once Earth is ice-free, this becomes the new normal for the climate system, and only a significant reduction in incoming solar energy or the amount of carbon dioxide (or other greenhouse gases) in the atmosphere will bring the climate back to its present-day condition.
Methane and the Ice-free Earth
Those two factors are complex enough on their own. But there’s a third tipping point: methane. As the climate warms, methane gas will be released from melting permafrost. There are tens of billions of tons of methane trapped beneath the Arctic permafrost . Over the course of a century, methane is a greenhouse gas 20 times more potent than carbon dioxide. This additional infusion of potentially vast quantities of greenhouse gases into the atmosphere will cause increased warming, which will melt even more ice and permafrost. This is yet another path to an ice-free Earth.
The Tipping Point
Is it too late? Are we past the tipping point and inevitably faced with an ice-free Earth? While observations confirm that the Earth is currently warming, it is not clear that we have reached the ice-albedo tipping point that simplified models describe. As in so many things, the trouble lies in the details. There are many other factors and feedbacks, both positive and negative, left unaccounted for in these climate models. According to the most recent IPCC report:
“Several components or phenomena in the climate system could potentially exhibit abrupt or nonlinear changes, and some are known to have done so in the past… For some events, there is information on potential consequences, but in general there is low confidence and little consensus on the likelihood of such events over the 21st century.” 
Although it’s important for us to continue to study potential tipping points in order to know what could happen, we should continue to base our decisions on the best-known science and observations. We must remain astutely aware of the possibility of tipping points within the climate system while tempering undue worry. Although tipping points are possible, they aren’t imminent, and time remains to address the climate problem.
Daniel Katzenberger is a Sustainability and Environmental Management graduate student in the Harvard University Extension School.
 National Aeronautics and Space Administration, Goddard Institute for Space Studies, “GISS Surface Temperature Analysis,” http://data.giss.nasa.gov/gistemp/graphs_v3/
 “Melting glaciers a ‘climate tipping point’, Bonn meeting told,’’ http://www.irishtimes.com/news/world/europe/melting-glaciers-a-climate-tipping-point-bonn-meeting-told-1.1831906
 Snowballearth.org, “Bibliography of Precambrian glacial deposits and post-glacial cap carbonates,” http://www.snowballearth.org/bibliography.html
 Wikipedia, “Snowball Earth,” http://en.wikipedia.org/wiki/Snowball_Earth
 Paul F. Hoffman and Daniel P. Schrag, “The snowball Earth hypothesis: testing the limits of global change,” http://onlinelibrary.wiley.com/doi/10.1046/j.1365-3121.2002.00408.x/abstract
 The Independent, “Methane meltdown: The Arctic timebomb that could cost us $60trn”, http://www.independent.co.uk/news/science/methane-meltdown-the-arctic-timebomb-that-could-cost-us-60trn-8730408.html
 Intergovernmental Panel on Climate Change, “Climate Change 2013 The Physical Science Basis” AR5 Report. p. 1033. http://www.ipcc.ch/report/ar5/wg1/mindex.shtml
Special thanks to MIT Professors Kerry Emanuel, Dan Cziczo, and David McGee for their edX course in Global Warming Science, which contributed to the author’s interest and further studies in this subject matter.
Additional thanks to Penn State Professor Richard Alley for his Coursera course in Energy, the Environment, and our Future, which helped the author to understand the relationship between the Earth’s climate and basic physics.