(Dutch version here)
James Hansen has put the concept of ‘tipping points’ on the agenda. It is not a strictly defined term, but at a tipping point, a relatively small change has a relatively large consequence, and the climate could end up in a different equilibrium state than before. (Compare it with the concept of meta-stability, with the classical example of a ball on a hill, which needs only a minor push to end up in the valley – the new equilibrium state.)
Large scale melting of ice could cause such a tipping point. Ice reflects a much larger part of the incoming solar radiation (i.e. it has a higher albedo) than land or water surfaces do. Therefore, when ice melts and the underlying land or water surfaces become exposed, much more sunlight will be absorbed than was previously the case. This causes more warming, which causes more melting, and the circle is closed.
The amount of Arctic sea ice at the end of summer has dramatically decreased over the last 30 years. The last two summers (2007 and 2008) ended with even smaller amounts of ice than would be expected based on the long term downward trend. It is too early to tell whether this means that the trend has changed (i.e. having passed a ‘tipping point’). Melting sea ice has no direct consequence for sea level, and if the warming trend is halted or reversed, the sea ice is expected to return to its ‘normal’ state. It is therefore a reversible tipping point.
Decrease of Arctic sea ice extent over the last 30 years. Data are for September, when the ice extent reaches its minimum. Source: NSIDC
The melting of land ice, on the other hand, does lead to sea level rise, and is practically irreversible on human time scales. The melting of Greenland would lead to a globally averaged sea level rise of about 6 meters. The West Antarctic ice sheet is good for about 7 meters, whereas the remainder of Antarctica has enough ice for over 50 meters global sea level rise. But that’s not gonna happen any time soon, is the expectation. No major changes are happening in the Antarctic, and in some places in the interior ice mass even seems to be increasing, due to increased snowfall. This is a predicted consequence of slight warming, because it leads to more water vapor in the air. As warming continues, melting will at some point start to outperform the effects of increased snowfall.
Sea level rise
According to Hansen et al, “equilibrium sea level rise for today’s 385 ppm CO2 is at least several meters, judging from paleoclimate history.” This seems predominantly based on the fact that in the previous interglacial, 125,000 years ago, sea level was about 6 metres higher than now, while the average temperature was about 1 degree higher. Even when the CO2 concentration would stop increasing, the Earth would still continue to warm up by another 0.5 degrees, mainly due to the thermal inertia of the oceans. So we’ll approach the same global average temperature of 125,000 years ago, even with current CO2 levels.
It’s not evident to what extent the relation between temperature and sea level is linear. Over ‘short’ timescales, when thermal expansion is the main influencing factor, it is probably close to linear. Several equilibrium situations from the distant past also show a strong relation over longer timescales, mainly influenced by the amount of land ice. The whole idea of ‘tipping points’ is of course that changes happen stepwise, rather than smoothly.
Relation between sea level (relative to today) and global average temperature based on different epochs. LGM stands for Last Glacial Maximum, Eocene is also known as PETM (Pleitocene-Eocene thermal maximum), Eemian is the previous interglacial. YBP stands for years before present. Numbers are from multiple sources and are associated with a ‘certain’ degree of uncertainty.
To what extent can we translate relations between climate variables from the past to the current situation? Melting of polar ice mainly depends on the regional temperature, and its relation with the global average temperature is not necessarily constant. We know relatively little about dynamical processes that influence the breaking up and melting of land ice. But apparently large changes in sea level are possible if the temperature remains long enough above (or below) a certain value. The examples from the past may give a sense of what order of magnitude sea level rise we could eventually expect for a given temperature increase. The rate of sea level rise is the most uncertain. Most scientific literature concludes that sea level rise won’t be more than one or at most two meters by 2100 (but it will continue to rise thereafter). That is quite a strong increase for large parts of the world to adapt to, and uncertainty in the rate and level of the rise is not really comforting. The examples from the past are even less so.