That the ice in the Arctic is melting faster than the rather conservative IPCC projection in the AR4 is obvious - in area, extent and volume. A nice recap of the 3 metrics here.
The effect of the ice melt in terms of albedo change, rise in sea levels etc are pretty well known. The most interesting effects, however, may not be any of these things. This is discussed in a recent paper by Shakun et al who were looking at the causes of the last deglaciation, probably the last time before now that the global climate changed in a hurry. A post at Real Climate summarise the issues.
It has long been known that characteristics of the Earth’s orbit (its eccentricity, the degree to which it is tilted, and its “wobble”) are slightly altered on timescales of tens to hundreds of thousands of years. Such variations, collectively known as Milankovitch cycles, conspire to pace the timing of glacial-to-interglacial variations.
Despite the immense explanatory power that this hypothesis has provided, some big questions still remain. For one, the relative roles of eccentricity, obliquity, and precession in controlling glacial onsets/terminations are still debated. While the local, seasonal climate forcing by the Milankovitch cycles is large (of the order 30 W/m2), the net forcing provided by Milankovitch is close to zero in the global mean, requiring other radiative terms (like albedo or greenhouse gas anomalies) to force global-mean temperature change.
The last deglaciation occurred as a long process between peak glacial conditions (from ~26-20,000 years ago) to the Holocene (~10,000 years ago). Explaining this evolution is not trivial. Variations in the orbit cause opposite changes in the intensity of solar radiation during the summer between the Northern and Southern hemisphere, yet ice age terminations seem synchronous between hemispheres. This could be explained by the role of the greenhouse gas CO2, which varies in abundance in the atmosphere in sync with the glacial cycles and thus acts as a “globaliser” of glacial cycles, as it is well-mixed throughout the atmosphere. However, if CO2 plays this role it is surprising that climatic proxies indicate that Antarctica seems to have warmed prior to the Northern Hemisphere, yet glacial cycles follow in phase with Northern insolation (“INcoming SOLar radiATION”) patterns, raising questions as to what communication mechanism links the hemispheres.
Also, there is the well known observation that CO2 levels measured in the Antarctic ice cores show CO2 rises well before the temperature signal rises - the temperature-lags-CO2 question. So what gives?
Antarctic ice core records...are not necessarily an accurate representation of global temperatures. In recent years there have been many studies collecting data from ice cores in Greenland, sediments drilled from the ocean floor and from continental lakes, and so forth. Most of these proxies don't extend as far back in time as the Antarctic ice cores, but many do extend back to the last glacial-interglacial transition which began approximately 18,000 years ago, as Figure 1 shows.
Shakun et al. examined 80 such proxy records from around the globe (Figure 2), recording sea surface temperatures for the marine records and surface air temperatures.
By comparing the atmospheric CO2 increase (note that since CO2 is well-mixed in the atmosphere, a single ice core record can be used as an accurate representation for CO2 - Shakun et al. used the Antarctic EPICA Dome C ice core for CO2 data) to these many different temperature records, Shakun et al. are able to discern whether the CO2 increase led or lagged temperature changes in various different geographic locations, and for the planet as a whole.
This is where it really gets interesting, because the answer is yes - CO2 lags and leads. In the Southern Hemisphere, Shakun et al. found that the temperature rise happened first, whereas in the Northern Hemisphere, the CO2 increase was first (Figures 3 and 4).
- As we already knew, the Earth's orbital cycles trigger the initial warming (starting approximately 19,000 years ago), which is first reflected at the highest latitudes (i.e. Greenland and the Arctic - see "Onset of seesaw" in Figure 4).
- This Arctic warming melted large quantities of ice, causing fresh water to flood into the oceans.
- This influx of fresh water then disrupted the Atlantic meridional overturning circulation (AMOC), in turn causing a seesawing of heat between the hemispheres. The Southern Hemisphere and its oceans warmed first, starting about 18,000 years ago.
- The warming Southern Ocean then released CO2 into the atmosphere starting around 17,500 years ago, which in turn caused the entire planet to warm via the increased greenhouse effect.
- In short, the initial warming was indeed triggered by the Milankovitch cycles, and that small amount of orbital cycle-caused warming eventually triggered the CO2 release, which caused most of the glacial-interglacial warming. So while CO2 did lag behind a small initial temperature change (which mostly occurred in the Southern Hemisphere), it led and was the primary driver behind most of the glacial-interglacial warming.
According to the Shakun et al. data, approximately 7% of the overall glacial-interglacial global temperature increase occurred before the CO2 rise, whereas 93% of the global warming followed the CO2 increase.
Now we seem to be on a similar deglaciaton path, with the fossil-fuel burn-led Arctic melt playing the role of Milankovitch cycle onset. It will be highly interesting to see whether the AMOC behaves as it did in the last deglaciation. In fact, these changes, once set in motion, could theoretically be irreversible, so that decreasing CO2 emissions that are planned for later in this century may not make too much of a difference.