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What do we really know about global warming?

Surface melting on the Greenland Ice sheet

Geoscientist 17.7 July 2007

Colin Summerhayes* takes a swipe at the warming deniers...

We live in a greenhouse world. Much of the sun’s energy is reflected back into space from the Earth’s surface as long-wave infrared radiation. While nitrogen and oxygen are transparent to this, carbon dioxide (CO2) and water vapour are not. They act like a blanket, absorbing some of the reflected infrared, and making the atmosphere warmer. Without them, the average temperature of the air at the surface of the Earth would be -18ºC. With them, this averages 14ºC, a difference of 32ºC. With its thicker atmosphere and many more such gases, the air temperature on planet Venus is 470ºC.

It is common sense and basic physics that adding more of any greenhouse gas to the atmosphere will increase temperatures. In 1895 the Nobel Prize-winning Swedish physicist Gustaf Arrhenius calculated that doubling the content of CO2 in the atmosphere would raise its temperature by about 5ºC, and he wasn't far out. The amount of CO2 in the atmosphere has ranged from 180 parts per million (ppm) in glacial times to 280 ppm during interglacials. We are living in the latest of these, but thanks to the burning of fossil fuel, CO2 now stands at 380ppm, a value not seen for more than a million years.

What temperature records show is an average increase in global surface air temperature of 0.7ºC since 1900. Of the last 12 years, 11 are among the 12 warmest since records began. This is entirely consistent with our understanding of atmospheric physics and the absorptive properties of greenhouse gases, which include not just CO2 but also methane (CH4), which has also increased at our hands from 715 to 1774 parts per billion. These are the underlying reasons for the warming we see – and contrary to the views expressed by Michael Crichton in his book State of Fear (2004) it can be shown conclusively that these temperature rises are not due to the ‘heat island effect’ in which major towns and cities influence the air around them.

The ocean has also warmed significantly - though only over its upper 3000 metres. The resulting thermal expansion has caused a steady rise in sea level of a few millimetres per year. The rise will continue until the ocean’s deep waters have also warmed. But averages are not the whole story. The climate system redistributes heat from the equator to the poles, and the observed warming is greater at the poles than elsewhere - especially in the Arctic, where the rate is double the global average. The Antarctic Peninsula has also warmed by a similar amount (5ºC in summer, in the past 50 years).

The surface area covered by Arctic sea ice has shrunk by 2.7% per decade, with more shrinkage in summer (7.4%) than in winter. The total loss averages 21% since 1979. Ships will soon sail across the Arctic Ocean. The temperature of the top of the permanently frozen ground (permafrost) in the Arctic has warmed by 3ºC, disrupting houses, roads and pipelines. Eighty seven percent of glaciers in the Antarctic Peninsula have shrunk significantly in recent years. The Larsen A and B ice shelves recently collapsed in the Weddell Sea, releasing the dam that held several glaciers in place, allowing them to speed up and lose mass more quickly.

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 CO2 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" CO2 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 CO2 has also been discredited. Volcanoes do contribute CO2 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 CO2 in the atmosphere. Then, the CO2 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 CO2 (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 CO2 rise. Our massive outpouring of CO2 means that temperature will now follow CO2, as it did in the past after the CO2 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.

Suggested further reading

Christoffersen & Hambrey: Is the Greenland Ice Sheet in a State of Collapse? Geology Today 22, 3 May/June 2006. Raises concerns about the parlous state of ice sheet models and the need to improve them to get at potential sea-level change.

Fookes & Lee, Climate variation: a simple geological perspective, Geology Today 23, 2 March/April 2007. Reminds the reader how external forcing by routine, regular and repeated changes in the Earth's orbit provides the pacemaker for the ice ages, the signal of which is well documented by the geological record.

* Executive Director, Scientific Committee on Antarctic Research (SCAR), Scott Polar Research Institute, Lensfield Road, Cambridge CB2 1ER, UK T: +44 (0)1223 336542; M: +44 (0)7834269997; F: +44 (0)1223 336549 E: