The Oxford Science Lecture Series

RUTH MCDONALD

Hadley Centre for Climate Prediction and Research

"Climate Studies from 1860 - 2100: Is our climate really changing?"

University Museum, Oxford, 28th October 1999

Another Oxford Science Lecture, the 11th in our series, was given on October 28 1999 by Ruth McDonald, a member of a research team at the Hadley Centre for Climate Prediction and Research, which is linked to the Met. Office. The subject of Ruth's lecture was "Climate Studies from 1860 to 2100 - is our climate really changing?" Somehow we were not altogether surprised that the short answer was, `Yes, we believe it is', but the route by which the professional meteorologist arrived at that conclusion was far more fascinating and credible than the short-cuts of distorted facts with which the media have been bombarding us in recent years.

It is difficult, from an inside position, to conduct studies on a body as large and diverse as the Earth, whose changing forces and evolution one is now trying to understand from a kind of hindsight. The key element is the atmosphere, which is responsible for trapping solar energy - chiefly through its consituent gases carbon dioxide, water and methane. These so-called greenhouse gases are generated naturally by the action of sunlight on plants and moisture everywhere, in a diurnal cycle, while mankind adds fossil-fuel and aerosol emissions (mostly from industry, power-stations and transport) at random times and in localized areas. Vegetation, through photosynthesis, absorbs carbon dioxide and releases oxygen, thus helping to promote a dynamic equilibrium between these atmospheric constituents. However, artificial additions of carbon dioxide and deforrestation will tend to exacerbate the overall situation wherever in the world they occur, since each triggers knock-on effects on a global scale. Actually, while increased volumes of greenhouse gases warm the lower atmosphere (troposphere), pollutants tend to cool the upper layers (stratosphere) by causing moist air to nucleate onto finer particles, forming more clouds which back-scatter sunlight, but the two effects do not cancel and neither should they be encouraged to.

Measuring the temperature of the Earth's surface demands methods which are as unbiassed as possible by short-term changes, local influences and human activity. Remote ice-core measurements show a slow temperature increase up to the middle of this century but a more dramatic one since - a nett rise of 0.6 C. Parallel to that rise are increases in greenhouse-gas contents (e.g. a 100% increase in methane), aerosols and pollutants. Such effects, if prolonged, will cause increased melting of Antarctic ice, with devastating consequences for low-lying countries. Tropical vegetation is also likely to decline, implying a reduced ability globally to absorb carbon dioxide. The latter has a long lifetime, and an actual fall in production levels is required in order to hold the global content at 1990 levels.

The group at the Hadley Centre builds a realistic world climate model and studies its behaviour under controlled experiments (e.g. by altering man-made contributions, population growth, industrial habits). It is essential to parameterize correctly all the natural causes of temperature variability, and to include accurately factors such as changes in the solar energy output with solar cycle, changes in the orbit of the Earth, and volcanic eruptions (the latter are particularly effective at climate disruption). A model of one million grid points is generated by subdividing the globe into squares, each modified to take account of local conditions; the higher the resolution, the better the model is able to predict details. The conclusion is that the observed 0.6 C increase in temperature is due to human activity. When present levels of concentration are extrapolated for the next 100 years, models predict a rise of 4 C in the temperature of the troposphere (of which 0.6 has already occurred), moderated only rather feebly by increased aerosol concentrations in the stratosphere. The surfaces of land masses would experience a greater increase (6 C) than deep water (3 C).

In detail, the changes show interesting patterns. Ice has a higher albedo (ability to reflect) than water, meaning that snow and ice fields tend to keep themselves cool whereas ice once melted will absorb heat relatively faster. Ocean temperatures will rise at different rates according to the local profile and depth. Air holds more moisture as it warms so increased precipitation will accompany rises in temperature, though here the Hadley models are less confident; models from different sites are less accordant, and only a qualitative statement can be made at present. Between 1860 and 2100, an increased global temperature will account for an increase of 0.7 m in mean sea levels: thermal expansion accounting for 0.4 m, an additional 0.2 m from melting glaciers, and 0.1 m from melting of the Greenland ice sheets. So far, 0.1 m rise has occured this century.

Ruth is also investigating associated changes in winter storms (low-pressure centres) occurring in the N. Atlantic, N. Pacific and Mediterranean Oceans. More intense storms are predicted, though slightly fewer in number, and more tropical cyclones are likely though as they are small-scale features the work requires very high-resolution models. One important finding is that their peak dates are likely to lag, e.g. from July to October in the NW Pacific, with obvious repercussions for local agriculture.

Ruth's lecture was fascinating and the message from the Met. Office was clear, though gloomy. However, we must not forget the basic difficulty of observing the Earth from an insider position. Some interpreters of measurements made from satellites are now claiming that there is no trend at all in recent world temperature... Fortunately there is a little time in hand while the scientists concerned sort it all out. But even for the sake of cleaner air and productive land alone, we will do well to heed the warnings.

Elizabeth Griffin.