Land ice in Greenland and Antarctica is melting faster. To some extent this loss of ice is balanced by an increase in snowfall from warmer, wetter air. Nevertheless, data collected by satellites show that these ice sheets are losing around 125 gigatonnes per year (two thirds of it from Greenland). That is equivalent to a sea-level rise of 0.35mm/year. The overall rise in sea levels averaged 2mm/year for much of the 20th Century, but increased to 3mm/year after 1990. This is believed due to a combination of thermal expansion of the oceans, plus the melting of the ice sheets, plus the melting of glaciers at all latitudes. Because CO
2 in the atmosphere is rising exponentially, changes in sea-ice retreat, permafrost melt, ice sheet melt, and glacier decay will probably be exponential too.
Our present numerical models of ice sheets and glaciers fail to take into account the dynamics of ice motion and melting – neglecting summer surface melting. This reduces the reflectivity of the ice surface, allowing surface meltwater to work its way to the glacier bed, thus lubricating it and further speeding glacier flow.
On 2 February 2007 the Intergovernmental Panel on Climate Change (IPCC) forecast that sea level would rise by less than 1m by 2100. This does not take ice dynamics into consideration, and so must be considered conservative. Experts like James Hansen, Director of NASA’s Goddard Institute for Space Science in New York, who have taken ice dynamics into consideration, forecast a rise of 5m by 2100.
Information on past climate change (from tree rings, ice and sediment cores) tells us that the warmth of the past 50 years can be described as "unusual" within at least the past 1300 years. The last time the polar regions were warmer than they are now (by about 2-3ºC) was 125,000 years ago during the last interglacial, when reductions in polar ice volume led to a sea level rise of 4-6m. Temperatures were high at that time due to the peculiarities of the Earth’s orbit. During the Pliocene (2-5Ma) before the Quaternary ice age began, temperatures were also significantly warmer than today. Ice caps were smaller and sea level was possibly as much as 20m higher than today.
If there were no "excess" CO
2 in the atmosphere at present, the Earth would be cooling slightly, as a function of its orbit around the sun. The sharp rise in warming we observe cannot be explained by anything other than the injection of greenhouse gases into the atmosphere by humans. The idea that this sharp warming is related to changes in the activity of the sun has been discredited. Solar irradiance has been measured by many means, including by satellite, and it neither varies enough to cause the temperature rise we observe, nor on the same timescale.
The idea that volcanoes are somehow responsible for the increase in CO
2 has also been discredited. Volcanoes do contribute CO
2 to the atmosphere, but only small amounts on average, and only during eruptions. They cannot have produced a continuous sharp rise in the period under consideration. Usually, major eruptions are associated with cooling because they release extra aerosol particles.
There is much local variability in the climate system. Most people have heard of the El Niño warm events that recur on average every four years. El Niño swings back and forth between one climate state and another (from warm El Niño to the cold La Niña). There are many similar regional systems, which oscillate on timescales of about a decade or two – the North Atlantic Oscillation, the Pacific Decadal Oscillation, The Indian Ocean Dipole, the Tropical Atlantic Dipole and so on. Since 1900, these various systems have provided “noise” around the basic underlying rising signal.
Some aspects of the climate system may oscillate at longer periods. One of these may have given rise to the so-called Medieval Warm Period (800-1300 AD) and to the Little Ice Age (1650-1850 AD). It is not easy to compare these events between hemispheres, as they tend to start 300-400 years earlier in the south than in the north. This lag may reflect the slow oceanic shunting of heat between hemispheres. The signals also tend to be stronger in the north than in the south. This asymmetry is true of today’s global warming, and may reflect the greater influence of northern hemisphere land (compared to southern hemisphere ocean) on atmospheric temperature.
The Little Ice Age peak appears to coincide with a period of diminished solar activity (the Maunder Minimum) in the northern hemisphere, but not globally. The present warming seems unnatural in comparison with these events, as it occurs simultaneously in both hemispheres – its signal has no lag.
If emissions stabilise at 2000 levels, then a further temperature rise of 0.6ºC can be expected by 2090AD, because of the ocean's slow response. If emissions continue to rise very modestly, a rise in global temperature of 1.8ºC is expected by 2090.
During ice ages, temperatures fluctuated considerably - as did CO
2 in the atmosphere. Then, the CO
2 followed the temperature signal. Tiny changes in the position of the Earth relative to the sun allowed the Earth to receive more solar radiation, raising temperature. Warmer temperatures then stimulated the growth of phytoplankton in the ocean. Plankton released CO
2 (by respiration and decay), further raising temperature to a level at which the climate stabilised in a new (warmer) state.
Many of these changes between warm and cold states during ice ages were extremely rapid: e.g. 10ºC in less than 50 years (perhaps in as little as 10 years). It is suggested that at first, the climate system reacts slowly to change before reaching a "tipping point" at which rapid and substantial change becomes inevitable. Current knowledge about ice sheet dynamics does not allow us to predict whether their behaviour will include tipping points as melting proceeds, but geological evidence suggests that it will. However, we now live in a different world from the ice ages of the past, when temperature rise triggered CO
2 rise. Our massive outpouring of CO
2 means that temperature will now follow CO
2, as it did in the past after the CO
2 rise had first been kicked off by an initial temperature hike.
If you took a roomful of 300 scientists involved in climate and polar studies and asked them whether they thought that we are dealing with anthropogenic global warming or some natural cyclic phenomenon, you would probably get no more than one in five to tell you that what we are seeing might possibly be natural.
I wish it were true that climate scientists are trying to frighten and delude everyone so as to get their research funded! That said, we must keep testing the global warming hypothesis. Indeed, the more it is tested, the more robust it proves. There are some things we still don’t know as much about as we would like – like the importance of clouds. But before such a detail causes you to dismiss climate forecasts out of hand remember those first principles – and think about Venus.