Does the Sun affect the Earth's climate?

Abstract

From Eric Priest, Mike Lockwood, Sami Solanki and Arnold Wolfendale Svensmark’s article in the February issue (AG according to Svensmark’s ideas, this produces fewer clouds and thereby heats the Earth. This raises two key questions: firstly, is this mechanism viable and, secondly, can George Bush gain comfort from it in terms of the origins of present-day climate change? It is now well established that, at least before 1970, there was indeed a correlation between the Earth’s climate and solar activity. This can be seen in the papers by e.g. Solanki and Krivova (2003), for the last 150 years and Bond et al. (2001) over longer timescales. For instance, in the 17th century (dubbed the “little ice age”) the temperature of northern Europe was lower than normal and the river Thames froze over, and this was also a time of very low solar activity (called the “Maunder minimum”) when there appear to have been very few sunspots at all for 70 years. Also, the concentration of CO2 has been measured for the past 650 000 years from ice cores: it decreases during each ice age and goes up in between, to a maximum value that is generally 280 parts per million. Although these correlations do exist, there is as yet no generally agreed mechanism to explain them. The white-light solar emission does vary, but by only 0.1% over a solar cycle, and the century-scale drift appears to be also of this magnitude, which is too small to explain the variation in the Earth’s temperature. One suggestion (not yet proved) is that the UV or EUV emission of the Sun, which varies much more than the white-light emission and which is absorbed high in the atmosphere, could somehow be influencing the lowatmosphere climate and amplifying the solar effect. Another plausible suggestion is the one offered by Svensmark: but it is also not yet proved, since it is at present not clear that the cosmic ray-induced aerosols are in practice strong enough to affect climate. A puzzling feature of the suggestion is that there is no correlation of cosmic-ray flux with high-level clouds – which is where the ionization is highest – and yet an apparent correlation at low levels (below 3.2 km) where the ionization is low. Another difficulty concerns the averaging that is needed, since both cosmic rays and cloud cover vary with latitude in different ways. So, even if it works, does Svensmark’s mechanism give any support to George Bush’s attitude that the current global warming is not caused by human activity? Not at all! The observed correlations between the Sun’s behaviour and the Earth’s climate have completely failed since the 1970s. In the past 30 or 40 years the Earth’s temperature has gone up much more rapidly than you would expect from the Sun – indeed there is strong evidence that since 1985 all the changes in the Sun have been in the opposite direction to that required to warm the Earth. Also, the amount of CO2 in the atmosphere is now 380 parts per million – very much larger than it has been for the past 650 000 years. This was highlighted at a recent meeting in the Royal Society on the science of climate change following on from the IPCC’s (Intergovernmental Panel on Climate Change) report reviewing the latest scientific data on global warming. The mechanism proposed by Svensmark and any influences that the Sun has had on our past climate are valid and interesting fields of study that may well give us valuable new understanding of our climate system. But, in our view, there is no doubt at all that the ongoing global warming is not being caused by the Sun but mainly by the greenhouse gases such as CO2 that we are emitting. We all, therefore, have a part to play in reducing the greenhouse effect in future – we must not fail to respond to the rapid and unprecedented changes that are taking place today because of debate over the much slower changes that occurred in the past. Eric Priest, University of St Andrews; and Mike Lockwood, Sami Solanki and Arnold Wolfendale References Bond et al. 2001 Science 294 2130. Solanki and Krivova 2003 J. Geophys. Res. 108 7.