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Carbon Brief Staff

Carbon Brief Staff

20.01.2012 | 1:48pm
ScienceQ: Compared to man-made climate change, how much will the Sun influence the climate? A: Not much.
SCIENCE | January 20. 2012. 13:48
Q: Compared to man-made climate change, how much will the Sun influence the climate? A: Not much.

Climate scientists have found that the 11-year solar activity cycle has only a small effect on global average temperature, compared to man-made climate change. Now another paper, in press and currently available on the Journal of Geophysical Research website, adds to the evidence supporting this conclusion.

The sun’s activity waxes and wanes on a roughly 11-year cycle. Since around the beginning of the 20th century solar activity has been relatively high, so scientists have labelled the period a ‘grand solar maximum’, or the ‘Modern Maximum’. There have also been ‘grand solar minima’ – periods where solar activity is unusually low. Perhaps the most notable of these is the ‘Maunder Minimum’, a 70-year period spanning 1645-1715 which saw virtually no sunspots. This is illustrated in the graph below:

NASA ssn

Source: Solar Physics Group at NASA’s Marshall Space Flight Center

Scientists think it probable that the Modern Maximum is nearing its end, and that we could even be heading towards another Maunder-style minimum, but predicting how solar activity might change in the future is notoriously difficult.

The prospect of a future grand solar minimum has led some to ask whether it could ‘buy us some time’ in mitigating against man-made climate change ( here and here for example), so it is important to fully investigate plausible future changes in solar activity and its impact on climate – hence this new study.

Researchers from the University of Reading and the Met Office used a simple climate model to determine how average global temperature might be affected by future solar cycle changes, assuming that they follow a similar pattern to the last 9,000 years.

Their results suggest that the solar cycle is likely to affect global surface temperature far less than man-made greenhouse gas emissions over the 21st century. Even the sun entering a new Maunder-style Minimum is likely to only give a relative cooling of 0.13 °C by the end of the 21st century, compared to warming from man-made greenhouse gas emissions of 2.55 °C, which is projected in the paper using a fairly low emissions scenario.

This conclusion supports the findings of previous research. For example, a 2010 Geophysical Research Letters paper found that a Maunder-style minimum would cause 0.3°C cooling by the year 2100, a small fraction of the warming expected from man-made greenhouse gas emissions over the same period.

Sadly the weight of scientific opinion on this issue hasn’t been enough to stop some particularly misleading stories being published about the sun’s impact on global temperature. Last year following mention of the solar cycle in a scientific press release, articles appeared in the UK press which bore little relation to the scientific findings they were reporting. We blogged about it on each occasion, see Factcheck: Christopher Booker and the missing ice age, Ice age! How the Daily Express use inverted commas to mangle science and Sunspots, ice ages, and the biggest experiment ever for more details.

These seem to be the result of misunderstanding the relationship between solar activity and climate change, and assuming that solar activity has a bigger effect on temperature than man-made climate change – a myth perpetuated by climate skeptic commentators. Misunderstanding can be compounded by confusing the ‘Little Ice Age‘ – a period of cooling affecting parts of the globe that lasted around 300 years, part of which coincided with the Maunder Minimum – with an actual ice age (a very different prospect).

Will this new research put paid to stories like this? Probably not! But this is a useful paper to remember the next time you spot a story which warns about an ‘impending ice age’ due to the sun’s activity.


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