There are some who assert that the global warming that has been observed on all continents is caused by changes in the output of the sun. This hypothesis does not stand up to scrutiny in either the short or the long-term, as made clear in James Hansen’s Storms of My Grandchildren as well as published papers of his, including “Target Atmospheric CO2: Where Should Humanity Aim?.” It is important to remember that what follows does not come from climate models, but rather from data on the paleoclimatic history of the planet, collected from ice and ocean cores and other sources.
The 12-year solar cycle
The sun dims and brightens across a twelve year cycle. While each square metre of the planet absorbs about 240 watts of sunlight averaged over day and night, the recorded magnitude of these cycles is about 0.2 watts. Not all forcings have the same effect on the climate. Taking the forcing caused by carbon dioxide (CO2) as the baseline, it can be calculated that the solar cycle forcing has an effective strength of between 0.2 and 0.4 watts. The climate forcing due to the 1750-2000 CO2 increase is about 1.5 watts. Other human-caused changes, such as adding methane, nitrous oxide, CFCs, and ozone to the atmosphere, make the total greenhouse gas forcing about 3 watts.
Each year, we are increasing the concentration of carbon dioxide in the atmosphere by about 2 parts per million (ppm). That equates to an effective forcing of 0.03 watts. As such, seven years of carbon dioxide emissions at the current level would offset the cooling effect of the sun being at the lowest ebb of its cycle. As a consequence, human-made climate change now overwhelms this natural cycle.
Long-term trends
Longer-term data also shows how greenhouse gases are more important to the climate than changes in solar output. The geological era spanning the last 65 million years is called the Cenozoic. Over that time, the sun’s output has increased by 0.4%. This corresponds to an increase of about 1 watt since the dinosaurs died out. Over this time period, the planet has actually cooled considerably: with mean global temperature more than 8°C higher at the end of the time of the dinosaurs. This, despite the increased solar output.
Over this timespan, the atmospheric concentration of CO2 has ranged from between 1,000 and 2,000 ppm during those hot years of the early Cenozoic and as little as 170ppm during recent ice ages. This range corresponds to a climate forcing of about 12 watts: at least ten times more than the forcings from the sun and from changes in the configuration of continents. As Hansen says: “It follows that changing carbon dioxide is the immediate cause of the large climate swings over the last 65 million years.”
The following diagram deserves consideration:
It shows temperatures from the Cenozoic: data that was obtained by examining the shells of microscopic animals called foraminifera. It shows the slow decline in mean global temperature over the whole span, as well as evidence that abrupt changes in temperature are possible.
What we’re doing now
One thing to consider is that if we keep increasing our greenhouse gas emissions, we will push carbon dioxide concentrations way above pre-industrial levels and into the range that existed at the beginning of the Cenozoic. While the cooling trend that we are living at the end of happened over tens of millions of years, temperature increases of well over 4°C could occur by the end of the century, with further warming beyond. While life has had ages to adapt to climate change as it was occurring before humanity, we are presiding over a spike in temperatures and greenhouse gas concentrations.
This graph shows CO2 concentrations from the last 400,000 years, as measured in ice core samples:
Keeping all that in mind, it seems very sensible to be working hard to keep the tip of that spike from getting too high. We should be worrying about our emissions, not blaming the warming we have observed on the sun and moving on.
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More on sun-climate relations
Four new papers discuss the relatiosnhip between solar activity and climate: one by Judith Lean (2010) in WIREs Climate Change, a GRL paper by Calogovic et al. (2010), Kulmala et al. (2010), and an on-line preprint by Feulner and Rahmstorf (2010). They all look at different aspects of how changes in solar activity may influence our climate.
The paper by Judith Lean (2010) has the character of a review article, summarizing past studies on the relationship between solar forcing and climate. The main message from her article is that the solar forcing probably plays a modest role for the global warming over the last 100 years (10% or less). It’s a nice overview, but I miss treatment of uncertainties.
Her analysis is based on the HadCRUT3 data, and I wonder if she would get similar results if she chose the GISTEMP or NCDC instead. The choice may in particular be relevant for the discussion of the temperatures after 1998.
Personally, I regard the data on solar activity before 1900 as quite uncertain too. The reason is that there are strange things happening to the solar cycle length in the shift from the 19th to the 20th century. Hence, any analysis based on the past centuries is uncertain because of suspect data quality in the early part of the record. Lean mentions that proxy-based records are uncertain, however.
Another source of uncertainty stems from the analysis itself – a regression analysis with chaotic data can easily yield misleading results. Gavin and I showed in a recent paper that multiple regression can produce strange results when applied to the global mean temperature and a number of forcings.
In other words, I think the reader may get the wrong impression from Lean (2010) that the link between solar activity and climate is better established than the data and methods suggest. Especially when she discusses forecasts for the near future (eg. for year 2014) – I fear that such a discussion can be misinterpreted and misused. However, that’s my view, and it does not necessarily mean that her paper is incorrect – quite the opposite, I think her main conclusions are sound (Her estimate of the solar contribution to the global warming over past century – 10% or less – is in good agreement with the figure Gavin and I got in our analysis).
The positive side is that the paper is probably clearer and more accessible without all these caveats. I also think she makes an interesting point when she discusses ‘fundamental puzzles’ associated with claims of strong solar role in terms of the past warming. She puts this into the context of climate sensitivity, arguing that it would imply that Earth’s climate be insensitive to well-measured increases in GHG concentrations and simultaneously excessively sensitive to poorly known solar brightness changes. Furthermore, Lean argues that it would also require that the Sun’s brightness increased more in the past century than at any time in the past millennium – a situation not readily supported by observations.
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So what can we learn from these articles? What we see is how science often works – increases in knowledge by increments and independent studies re-affirming previous findings, namely that changes in the sun play a minor role in climate change on decadal to centennial scales. After all, 2009 was the second-warmest year on record, and by far the warmest in the southern hemisphere, despite the record solar minimum. The solar signal for the past 25 years is not just small but negative (i.e. cooling), but this has not noticeably slowed down global warming. But there are also many unknowns remaining, and the largest uncertainties concern clouds, cloud physics, and their impact on climate. In this sense, I find it ironic that some people still rely on the cosmic rays argument as their strongest argument against AGW – it does involve poorly known clouds physics!