It’s all to do with the sun – for example, there is a strong link between the increased temperature on earth and the number of sunspots on the sun.
But what does the science – and the Royal Society – say?
Change in solar activity is one of the many factors that influence the climate but cannot, on its own, account for the change in global average temperature that we have seen in the 21st century.
Changes in the sun’s activity influence the earth’s climate through small but significant variations in its intensity. When it is a more “active” phase – as indicated by a greater number of sunspots on its surface – it emits more light and heat. While there is evidence of a link between solar activity and some of the warming in the early 20th century, measurements from satellites show that there has been very little change in underlying solar activity in the last 30 years. There is even evidence of a detectable decline – and so this cannot account for the recent rises we have seen in global temperatures. The magnitude and pattern of changes to temperatures can only be understood by taking all of the relevant factors – both natural and human – into account. For example, major volcanic eruptions produce a cooling effect because they blast ash and other particles into the atmosphere where they persist for a few years and reduce the amount of the sun’s energy that reaches the earth’s surface. Also, burning fossil fuels produces particles called sulphate aerosols which tend to cool the climate in the same way. Over the first part of the 20th century higher levels of solar activity combined with increases in human generated carbon dioxide to raise temperatures. Between 1940 and 1970 the carbon dioxide effect was probably offset by increasing amounts of sulphate aerosols in the atmosphere, and a slight downturn in solar activity, as well as enhanced volcanic activity.
During this period global temperatures dropped. However, in the latter part of the 20th century temperatures rose well above the levels of the 1940s. Strong measures taken to reduce sulphate pollution in some regions of the world meant that industrial aerosols began to provide less compensation for an increasing warming caused by carbon dioxide. The rising temperature during this period has been partly abated by occasional volcanic eruptions.
Below are two further responses.
What people argue:
“Carbon dioxide only makes up a small part of the atmosphere and so cannot be responsible for global warming.”
What the science says
Carbon dioxide only makes up a small amount of the atmosphere but even in tiny concentrations it has a large influence on our climate.
The properties of greenhouse gases such as carbon dioxide mean that they strongly absorb heat-a fact that can easily be demonstrated in a simple laboratory experiment. While there are larger concentrations of other gases in the atmosphere, such as nitrogen, because they do not have these heat trapping qualities they have no effect on warming the climate whatsoever.
Water vapour is the most significant greenhouse gas. It occurs naturally although global warming caused by human activity will indirectly affect how much is in the atmosphere through, for example, increased evaporation from oceans and rivers. This will, in turn, cause either cooling or warming depending on what form the water vapour occurs in, such as different types of clouds or increased humidity.
Humans have been adding to the effect of water vapour and other naturally occurring greenhouse gases by pumping greenhouse gases such as carbon dioxide into the atmosphere through, for example, the burning of fossil fuels and deforestation. Before industrialisation carbon dioxide made up about 280 parts per million of the atmosphere. Today, due to human influence it is about 385ppm. Even these tiny amounts have resulted in an increase in global temperatures of 0.74c.
What people argue:
“Rises in the levels of carbon dioxide in the atmosphere are the result of increased temperatures, not the other way round.”
What the science says
It is true that the fluctuations in temperatures that caused the ice ages were initiated by changes in the Earth’s orbit around the Sun which, in turn, drove changes in levels of carbon dioxide in the atmosphere. This is backed up by data from the ice cores which show that rises in temperature came first, and then were followed by rises in levels of carbon dioxide up to several hundred years later. The reasons for this, although not yet fully understood, are partly because the oceans emit carbon dioxide as they heat up and absorb it when they cool down and also because soil releases greenhouse gases as it warms up. These increased levels of greenhouse gases in the atmosphere then further enhance warming, creating a ‘positive feedback’.
In contrast to this natural process, we know that the recent steep increase in the level of carbon dioxide-some 30% in the last 100 years- is not the result of natural factors. This is because, by chemical analysis, we can tell that the majority of this carbon dioxide has come from the burning of fossil fuels. And, as set out in ‘misleading argument 1’, carbon dioxide from human sources is almost certainly responsible for most of the warming over the last 50 years. There is much evidence that backs up this explanation and none that conflicts with it.