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The science of global warming; Climate change: Evidence of climate change is based on years of research and thousands of critically reviewed scientific studies, as Professor Ian Hall of Cardiff University explains.

Byline: Professor Ian Hall

THE key to understanding global climate change is to first understand what the Earth's climate system is, how it operates and the changes it is capable of.

The Earth's climate system is a complex interaction between the atmosphere, the oceans and ice sheets (the cryosphere), living things (the biosphere), and soils, sediments and rocks.

These components all affect how the heat energy from the sun is redistributed around the globe from warmer to colder places.

This global movement of heat dris ves our weather systems, and we experience the effects as the 'average weather' across Wales.

This input is balanced by an output as the Earth's surface warms and radiates heat energy back into space.

If the Earth was in perfect energy balance, the temperature averaged across the planet's surface would be -19[bar]C - much colder than the typical surface temperature we currently enjoy (around 14C).

The reason the Earth's surface is warm enough to sustain life is the presence of greenhouse gases within the atmosphere.

These gases, which include water vapour, CO and methane, have a direct effect on the amount of heat energy that is trapped within the lower atmosphere, rather than escaping into space. If the concentration of CO in the atmosphere doubles, the 'resulting global average warming will very likely be between 2-4C - a good bet would be 3C, with the remaining uncertainty due to additional climate interactions, known as feedback effects.

Climatologists worldwide are working on reducing these smaller uncertainties, and predicting the regional effects of climate change, while the overall picture of global warming as a result of increased CO is inescapable.

The concentration of atmospheric CO has risen sharply over the past 150 years, from a pre-industrial value of 280 parts per million (ppm) to 385ppm today.

In 1850, levels of atmospheric CO were similar to those of the warmest intervals of the past 700,000 years.

Current levels of atmospheric CO are higher than they have been for several million years, and are similar to those in a very ancient time, when Earth's climate was warmer, and sea levels were significantly higher than today.

The current rise in CO is caused entirely by human activity and is primarily the result of burning fossil fuels, with a smaller contribution due to deforestation.

We know this because many independent observations show that the carbon content has also increased in both the oceans (leading to ocean acidification) and the land biosphere (after subtracting deforestation).

The ocean and land biospheres are the only stores of carbon that could provide a natural source of CO to the atmosphere on century timescales. Had they done so, they would currently hold less carbon, not more as is observed.

Observations of the climate system are crucial to establish actual climate trends.

In addition, climate models are used to predict how values such as global average temperature or sea level may be expected to respond in the future.

Since the beginning of the 20th century, global average climate has warmed by about 0.8C. Temperatures during the last decade have been the highest since measurements began in the mid-19th century and for many centuries before that.

Last year was likely the fifth-warmest year on record. Most of this warming is due to the rising concentration of CO and other greenhouse gases.

Other potential causes of the measured temperature trend, such as changes in the strength of the sun, volcanic activity, cosmic rays, or variation in the orbit of the Earth around the sun, are well observed, but fail to show changes capable of explaining the observed warming.

The energy from the sun has been directly measured by satellites since 1978. This data shows variations with time (the so-called 11-year solar cycle), but no trend that can explain Earth's recent warming.

There are also many ways scientists can infer the sun's output pre-1978 (like sunspot numbers, solar cycle length, radiocarbon data and more).

Although there are differences in the amount of variability estimated for earlier times, these records consistently indicate that during the 20th century the sun's output probably increased up to 1940, but has not significantly increased during the last 65 years.

As a result, this cannot explain the measured global warming.

Indeed, our most sophisticated climate models driven only by natural factors, such as those listed above, but without any manmade greenhouse gas forcing, show cooling in the latter half of the last century.

The measured global warming is only reproduced in the models when man-made greenhouse gas emissions are included.

Furthermore, the physics described above tells us that these CO emissions would have produced more warming than was observed, were it not for the competing cooling effect of increasing levels of atmospheric smog (aerosol pollution).

It follows that a continued increase in atmospheric CO will lead to a further rise in global average temperature. Following a range of reasonable assumptions about the future rates of our greenhouse gas emissions (excluding international mitigation agreements) the Intergovernmental Panel on Climate Change predicts that by 2100 this rise will reach about 2-6.5C above 1980-1999 levels.

As a comparison, the last major natural warming event occurred some 15,000 years ago at the end of the last great Ice Age, when the planet warmed by 5C over 5,000 years.

Before this event, most of Wales lay under an ice sheet several hundred metres thick.

Without intervention, our human-induced warming could force the climate system to warm by a similar amount over a much shorter interval.

Within average global warming, regional differences will be considerable and some areas will warm substantially more than others and certain areas may even cool.

The decades of critically reviewed research on the mechanisms and evidence of global climate change has developed into an extraordinary scientific consensus, and has informed the statements of many national and international bodies that have again a nd again come to the same conclusions - namely that human induced global climate warming is a real phenomenon. Professor Ian Hall is a palaeoclimatologist and member of the Palaeoclimate and Climate Systems Research Group at Cardiff University

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Variations in solar strength and activity cannot explain global warming Global climate expert Professor Ian Hall
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Title Annotation:Features
Publication:Western Mail (Cardiff, Wales)
Date:Jan 27, 2010
Words:1035
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