This fairly straightforward, if not obvious, research paper published in Geophysical Research Letters, has attracted a modest amount of media comment mostly as a result of the press release associated with it.
Many kinds of observations show that the Sun is in the deepest and longest activity minimum for almost a century. Satellite measurements confirm that solar radiation has never been weaker since records started in the 1970s. Some solar physicists have suggested it could indicate the beginning of a new Grand Minimum comparable to the Maunder Minimum in the late 17th century that was connected to the so-called Little Ice Age.
The press release said: “A new Grand Minimum of solar activity would decrease the rise of global mean temperature caused by human greenhouse gas emissions only marginally. A new modeling study by researchers of the Potsdam Institute for Climate Impact Research, published online today in the journal Geophysical Research Letters, finds a temperature offset of at most 0.3 degrees Celsius until the end of the century. This is less than ten percent of the temperature rise projected under “business as usual” scenarios of the Intergovernmental Panel on Climate Change (IPCC).”
“The notion that we are heading for a new Little Ice Age if the Sun actually entered a Grand Minimum is wrong,” says Georg Feulner, lead author of the study. “In fact,” he adds, “a minimum of solar activity would not prevent strong future warming if emissions of greenhouse gases continued at current levels.”
The gist of the paper is that, despite what happened in the past, if the sun repeated the behavior that led to the Little Ice Age then because of the much bigger influence of AGW the outcome would be very different.
To explore the effect of a 21st-century Grand Minimum Feulner and Stefan Rahmstorf used climate models with two assumptions about the future rise of greenhouse gasses. They performed three simulation experiments with different solar behaviour: one without a Grand Minimum, repeating the last 11-year solar activity cycle until 2100, and two with the Sun entering a new Grand Minimum lasting till the end of the century. In these minima, solar irradiance is reduced by 0.08 and 0.25 percent of its value in 1950, the former value corresponding to the reconstructed reduction of solar irradiance during the Maunder Minimum.
“Most likely, a new Grand Minimum of solar activity would diminish global mean temperatures in the year 2100 by about 0.1 or 0.2 degrees Celsius,” says Stefan Rahmstorf. Even taking into account all uncertainties in the temperature reconstruction, the forcings, and the model physics, the overall uncertainty is estimated to be at most a factor of three, so the solar cooling effect would very likely not exceed 0.3 degrees.
“Current temperature data also confirm that the effect of low solar activity on the climate is very small”, notes Rahmstorf. The current minimum has not noticeably slowed down global warming. Over the past 30 years temperatures have increased at a steady rate of 0.16 degrees Celsius per decade. According to the surface temperature analysis by NASA’s Goddard Institute for Space Studies, the year 2009, despite the solar minimum, was the second-warmest year on record globally, beaten only by 2005, and by far the warmest in the southern hemisphere. The month of January 2010 was the second warmest January on record globally, beaten only by January 2007.
Qualitatively the conclusions reached by this paper are somewhat obvious. If the Maunder Minimum had a temperature reduction of 0.4 deg C (the figure used in the paper) and future projected increases due to AGW are expected to be about 4 degrees then the sun’s influence will be minor. When the climate models used in the paper are run the sun’s influence is even less, about 0.1 deg C. However, as is often the case with models, there are complications, limitations and conflicts when placed alongside real-world data.
I would disagree with press release that the current solar minimum has not noticeably slowed down global warming. Something has obviously been preventing the world from warming since 1995 and the scientific literature is full of suggestions as to the cause. Although no one can come up with a workable physical mechanism yet it remains an interesting coincidence that after solar activity last peaked in the late 1980’s the global temperature hasn’t gone up much and has remained on a hiatus whilst the sun has gone through it prolonged minimum. So whilst the climate models don’t reproduce any convincing link it remains a suspicious coincidence.
It should also be noted that radiometers have measured a change in total solar irradiance of 1.3 Watts per square meter at the top of the atmosphere over the past few 11-year solar cycles. This is equal to the climate forcing effect of increasing greenhouse gasses accumulating in the earth’s atmosphere over the rising period of the solar cycle. This is an effect that may have an increased importance during the shorter than average sunspot cycles seen in the 20th century.
The rest of the press release’s comments refer to weather and not climate.
Indeed, this research paper acknowledges the downturn in solar activity but not the coincident hiatus in the earth’s temperature rise.
Another general point to be made is that the paper says that the solar contribution to the recent warming spell is negligible. As an aside, what I wonder was the sun’s contribution to the Medieval and Roman Warm Period which some scientists think was as warm or warmer than current conditions?
The researchers acknowledge that their work has limitations. Specifically it does not include details of Ultra Violet radiation from the sun which has a far stronger variation over the solar cycle than its visible light and with a different phase. It could have a discernable climatic influence by an unknown mechanism.
The climate model used in the paper predicts an average temperature rise of 0.23 and 0.20 deg C per decade between 2000 – 2030. However, since it is not mentioned in the paper that the average rise observed between 2000 and 2010 is zero this implies a 0.33 deg C per decade rise for the nest two decades. This means that the next 20 years will see more warming in absolute terms and at a faster rate than in the post – 1880 thermometer period. Since we have had only about 0.4 deg rise in the past 70 years I maintain that, whatever the models predict, such an increase is unlikely.
To summarise; This paper says that the sun will not save us from global warming. However, one could add a coda. Since 1995 something has.