Human evidence: The Earth’s energy balance
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Scientists know from basic physics that carbon dioxide and other greenhouse gases have a warming effect on the Earth, and that increasing amounts of these gases in the atmosphere – together with the associated feedback effects – would be expected to cause an increase in global temperature. Scientists predicted this global warming effect over 100 years ago – before human greenhouse gases emissions increased to the substantial levels seen today.
Some of the energy the Earth receives from the Sun is reflected back into space, some is absorbed by the atmosphere, and the remainder warms the land and oceans. The Earth’s surface gives off heat energy, a portion of which is absorbed by greenhouse gases and clouds in the atmosphere, delaying its progress out to space. Incoming solar energy and outgoing heat energy are the main components of the Earth’s ‘energy balance’. Any change in this balance results in the Earth warming up or cooling down until a new balance is achieved. These changes cause an increase or decrease in the amount of energy in the Earth’s lower atmosphere, which in turn triggers changes in the climate and associated weather.
Earth’s surface energy balance is determined by the difference between absorbed solar energy and outgoing heat energy. If the ‘absorbed’ component is larger than the ‘outgoing’, the resulting accumulation of energy causes the surface to warm. Many factors can influence the surface energy balance, including variations in the Sun’s output, the amount of sunlight reflected by the Earth’s atmosphere and surface, and changes in concentrations of greenhouse gases in the atmosphere. By measuring these effects, scientists can calculate the overall energy balance and estimate the rate at which global surface temperature will increase or decrease, as well as the temperature at which the planet will ultimately settle when the new balance is reached.
The amount of solar energy reaching Earth’s surface is determined by the output of the Sun – which varies on an assortment of timescales – and by the amount of sunlight reflected by the planet’s surface and atmosphere back into space. The proportion that is reflected is known as the planet’s albedo. Volcanic eruptions can increase albedo by ejecting gases which form aerosols, leading to a decrease in the solar energy reaching the surface. However, measurements show no significant trend in total solar output over the last 50 years and the change owing to volcanic eruptions is over a short time scale (about two years). This causes scientists to conclude that the incoming component of the energy balance has remained relatively stable in recent decades.
The amount of heat energy escaping to space is affected by concentrations of greenhouse gases in our atmosphere. According to physical principles, higher greenhouse gas levels cause an enhanced greenhouse effect while lower levels lead to a diminished one, both of which will change the Earth’s energy balance. Measurements show that carbon dioxide and other greenhouse gases have been accumulating in the atmosphere. By analysing different isotopes of carbon, and also by calculating the amount released as a result of burning known amounts of fossil fuel and clearing known areas of forest, scientists have shown that it is almost certain that the increase is due to human activities.
A stronger – or ‘enhanced’ – greenhouse effect causes more outgoing heat energy to be absorbed in the lower atmosphere. This changes the ‘outgoing’ component of the Earth’s energy balance, causing more heat to accumulate near the planet’s surface, manifesting itself as a warming effect, until the system readjusts and a new balance is reached. This conclusion is based on physical principles governing the behaviour of our planet’s atmosphere. More than 100 years ago, scientists predicted that an increase in the levels of greenhouse gases in the atmosphere would lead to a rise in global surface temperature. Their pioneering calculations have since been confirmed by scientific experiments and more detailed calculations based on our improved understanding of the behaviour of the climate system.
Many different factors influence the Earth’s climate. Professor Graf’s work focuses on a range of these factors, from human activity to volcanic eruptions. ‘Understanding what drives our climate and its variability is both fascinating (just look at clouds or volcanic eruption plumes) and essential to the society.’ Professor Graf’s work has resulted in the first successful simulation of the climate effects of a big volcanic eruption. However he’s quick to emphasise the complexity involved in such work. ‘There are always struggles with the big computer models and our work would benefit from more reliable observations. However, our understanding of the basic physics involved in the field is quite good.’