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Climate Hustle

Solar eclipse: Why the sun is not responsible for recent climate change

Posted on 21 August 2017 by Zeke Hausfather

This is a re-post from Carbon Brief

With a total solar eclipse sweeping across North America, everyone is suddenly paying attention to the sun. One of the most common sceptical arguments against human-caused climate change is that changes in solar activity, rather than just CO2, is playing the biggest role.

At first glance, it seems to make intuitive sense: the sun is a massive nuclear fusion reactor a million times larger than Earth, it is responsible for almost all the energy reaching our planet, and in the past few decades scientists have learned that solar activity varies significantly over time. Indeed, changes in the distribution of sunlight reaching the Earth clearly change the temperature dramatically on a daily and annual timescale.

However, since 1970 global temperatures have shot up by almost 0.7 C, while the amount of solar energy reaching the Earth has actually declined. Similarly, the upper atmosphere is cooling while the lower atmosphere warms, a clear fingerprint of warming from greenhouse gases rather than the sun.

This evidence suggests we can rule out a major solar influence on recent warming.

Moving in opposite directions

Satellites have been directly monitoring the amount of the sun’s energy reaching the Earth since the late 1970s. Before that, researchers kept careful records of the number of “sunspots”, dark patches on the surface of the sun that are strongly related to solar output.

Combining older sunspot measurements with modern satellite observations, scientists have put together estimates of the sun’s output – called Total Solar Irradiance, or TSI – for the past 400 years. There is still some debate around how much solar output changed in the past, with some reconstructions suggesting more variation than others.

The plot below shows the solar TSI reconstruction from Naval Research Laboratory Total Solar Irradiance-2 (NRLTSI2) model (red line) – one of the ones with larger changes in solar output over the past 150 years – alongside the surface temperature record jointly produced by  the UK Met Office Hadley Centre and the Climatic Research Unit at the University of East Anglia (black line).

Annual global mean surface temperature from HadCRUT4 in black in degrees C, along with NRLTSI2 TSI data in watts per metre squared. Lowess-smoothed curves are shown via dashed lines. Chart by Carbon Brief using Highcharts.

The temperature and TSI values shown in this plot are not intended to be directly comparable. Dr Gavin Schmidt, director of NASA Goddard Institute of Space Studies, tells Carbon Brief that it would take around a seven watts per metre squared change in TSI to result in 1C warming, meaning that the observed rise in solar activity between 1880 and 1980 would result in only 0.1C warming.

There are indirect effects that need to be accounted for as well. Increases in solar UV output increase the formation of ozone, which is itself a greenhouse gas. This could at most double solar’s contribution to warming, taking it to 0.2C.

This shows that solar has only had a small impact of global temperatures over the last century or so, says Schmidt:

“Just as for the other drivers of change (such as deforestation, air pollution and greenhouse gases) we can calculate the fingerprint for these variations in time throughout the climate system from the surface to the mesosphere [in the upper atmosphere]. When we match that up with what we have observed over time though, the solar contribution is small –- close to zero over the last 50 years, and perhaps as large as 10% since the late 19th century.”

A slight decrease in solar activity

It is also worth noting that almost all of the minor warming contribution from fluctuations in the sun’s output occurred before 1970.

In recent decades, when global temperatures have risen most steeply, TSI has been flat or even slightly declining, says Schmidt:

“The period of greatest warming – since about 1975 – has coincided with a slight decrease in solar activity.”

Solar output in the satellite era, when we have much higher confidence in the records, is shown in the figure below. Each black dot represents daily total solar irradiance, while the red line is a longer-term average.

Satellite-based daily Total Solar Irradiance (TSI) in watts per metre squared from 1976-2016 via PMOD. Red line represents a lowess-smoothed fit. Chart by Carbon Brief using Highcharts.

There is a clear cycle in solar activity of around 11 years. This has some effect on short-term climate, though it tends to average out over longer time periods. For example, the unusually low solar output in after 2009 may have contributed to slower warming of the Earth’s surface between 1998 and 2013.

Atmospheric ‘fingerprint’

The lack of an increase in solar activity after 1970 is one major argument against a solar role in modern warming. Another is the cooling in a layer of the upper atmosphere known as  the stratosphere.

If changing solar output were driving warming, the whole atmosphere would warm due to the addition of incoming solar radiation. In contrast, if warming was due to greenhouse gases, the lowest layer of the atmosphere – the troposphere – and surface would warm, while the upper atmosphere would cool as greenhouse gases trapped heat and prevented it from escaping the troposphere.

The figure below shows what has actually happened since 1950. The upper chart shows a clear cooling trend in the stratosphere, while the lower chart shows the warming of the troposphere and Earth’s surface. This is the “fingerprint” of warming from CO2 and other greenhouse gases, though recent declines in stratospheric ozone also play a large role in stratospheric cooling.

Global lower stratosphere (top) and troposphere (bottom) temperatures between 1958 and 2012. Figure from the UK MET Office HadAT. The CO2 cooling signal is clearer and more differentiable from ozone-driven stratospheric cooling in higher levels of the stratosphere.

Schmidt describes this evidence as “the biggest mismatch”. He tells Carbon Brief:

“If the warming at the surface was related to solar forcing, the upper atmosphere would also be warming. But it hasn’t been – it has been cooling, exactly as predicted from the effects of CO2 increases.”

Grasping at cosmic rays

With no discernable increase in solar activity, proponents of solar influence on modern warming have turned to other possible explanations. One that has gained traction in recent years is the idea that galactic cosmic rays may play a role in the Earth’s climate.

Galactic cosmic rays (GCRs) are high energy particles from beyond our solar system that regularly bombard the Earth. When solar activity is high, the “solar wind” – a stream of particles emitted from the sun – acts to reduce the number of GCRs that enter the Earth’s atmosphere.

Some research has found that GCRs in the atmosphere can play a role in cloud formation, with higher levels of GCRs potentially leading to more low-altitude clouds. These low-altitude clouds can influence the Earth’s climate by reflecting incoming sunlight back into space.

This has led some to suggest that changes in solar activity could influence the Earth’s climate by changing cloud formation.

However, the GCR hypothesis suffers from the same fundamental problem as total solar irradiance: it is moving in the wrong direction. Since 1960, the amount of GCRs reaching the Earth has increased, as shown in the figure below. If GCRs were a major influence on climate, this would result in cooling, not warming, over the past 50 years.

Daily Galactic Cosmic Ray (GCR) counts from the neutron monitor at the McMurdo Station in Antarctica. Yellow line represents a lowess-smoothed fit. Chart by Carbon Brief using Highcharts.

Other recent research has also cast doubt on the role of GCRs as a major factor in the Earth’s climate, suggesting that their effect on cloud formation is far too weak to make much of a difference.

Taken together, these three lines of evidence show that solar variations are not driving modern climate change, says Dr Mike Lockwood, a solar physicist at the University of Reading. As he concludes for Carbon Brief:

“Most-importantly, the observed height profile is all wrong: a total solar irradiance increase would drive higher temperatures at all heights in the atmosphere, not just the surface, whereas the data show that while the lower atmosphere (and surface) have warmed, the stratosphere has cooled. The only mechanism that predicts this is greenhouse gas trapping of thermal infrared radiation from Earth’s surface.

Secondly, the long-term trend in solar activity since 1985 has been weakly downward whereas that in surface temperatures has been a strong rise. This also eliminates other proposed mechanisms, such as high cloud production by cosmic rays.

And the third is that you simply cannot put the observed levels of CO2 into the atmosphere without causing warming of about the magnitude observed.”

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Comments

Comments 1 to 7:

  1. Good info! Also:

    Nighttime termperatures have risen faster than daytime temperatures. The sun doesn't shine at night, so that is not consistent with the sun being the cause. It is consistent with excess atmospheric CO2 being the cause.

    Winter temperatures have risen faster that summertime temperatures. The sun shines less in winter, so that is not consistent with the sun being the cause. It is consistent with excess atmospheric CO2 being the cause.

    Temperatures at the poles have risen faster than temperatures in temperate regions. The poles receive less sun, so that is not consistent with the sun being the cause. It is consistent with excess atmospheric CO2 being the cause.

    If the sun were causing the increase in temperature, the amount of energy the Earth radiates into space would go up as the planet warms. Satellites in space have measured a reduction in the energy the Earth radiates into space. The reduction is at the wavelength that is absorbed by CO2 in the atmosphere, and tracks the increase in CO2 in the atmosphere. This would not occur if the sun was causing the temperature increase, and can only be explained by increased CO2 from the burning of fossil fuels causing the Earth to warm by trapping heat energy before it can be reradiated into space, a process known as the "greenhouse effect".

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  2. Well if there are more pollutants it seems TSI is going to be less (more solar energy blocking). If greenhouse gases are greater then the atmosphere would warm more, so "relatively cooler ground and warmer air" would sound reasonable although all of it is heating up. As always, if one can form low clouds in low latitudes it helps. Idea: People are becoming more concerned about where to put old solar panels. If you put them in greenhouses (to absorb solar energy) in the deserts and pump seawater into the greenhouses you would do better than having a sand bottom in the greenhouse because sand reflects energy back out (is light coloured) of greenhouses. With other solar panels, coat them to make mirrors out of them and reflect solar energy into the greenhouses. Of put dark solar panels in shallow pools of seawater and reflect solar energy into the pools with the mirror solar panels to cause evaporation and more clouds and rain.

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  3. the upper atmosphere would cool as greenhouse gases trapped heat and prevented it from escaping the troposphere

    Is this correct? It makes intuituitve sense, but it's been my understanding that the full explanation is far more complex, and that this is not the primary contributor to the cooling of the stratosphere. If I recall correctly, Gavin Schmidt got caught by this many years ago in RealClimate, and had to rewrite big chunks of a post.

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  4. Regarding #3, the reason the stratosphere cools with increasing greenhouse gas concentrations is simply that it emits more IR radiation. Since there's little overlying gas to radiate back down, the result is a net cooling. This is extremely well understood, and was predicted by Manabe and Weatherald 50 years ago: http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281967%29024%3C0241%3ATEOTAW%3E2.0.CO%3B2 

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  5. Regarding #3: Sorry, here's the link to the classic paper on tropospheric warming and stratospheric cooling from 1967.

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  6. Swayseeker

    "People are becoming more concerned about where to put old solar panels. If you put them in greenhouses (to absorb solar energy) in the deserts and pump seawater into the greenhouses"

    Old solar panels won't power pumps because they are worn out and inefficient.  They are better for the rubbish dump or recycling.

    Why do greenhouses need sea water? Seawater kills most plants. Are you thinking solar distillation or something?

    "you would do better than having a sand bottom in the greenhouse because sand reflects energy back out (is light coloured) of greenhouses."

    The idea of greenhoses is to maximise heat gain, so why would you want to reflect the heat back out? Am I missing something?

    "With other solar panels, coat them to make mirrors out of them and reflect solar energy into the greenhouses."

    It would be much easier and cheaper to just use new mirrors.

    "Of put dark solar panels in shallow pools of seawater and reflect solar energy into the pools with the mirror solar panels to cause evaporation and more clouds and rain."

    Negligible effect. Rain is also not caused by evaporation as such.

    There is a proposal for massive solar electricity production in the deserts of north africa due to the phenomenal sunlight hours. Google Desertec. Its not without some challenges!

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  7. chrisd3 @3,

    The RealClimate problem post on stratospheric cooling was actually one of the launch posts for the site back in 2004 and was edited more than once before being replaced and declared "obsolete and wrong in many respects." There is a SkS post explaining why increased GHGs result in stratospheric cooling (a post which I see also underwent post-publication revisions). The reason why stratospheric cooling is being mentioned here because there are other factors affecting stratospheric temperature including TSI. This added complication allows the denialist message to create a fair amount of garbled nonsense on the subject, a process which actually itself causes increasing heat within the the deniosphere. Mind, while the deniosphere may relish the occasion of the likes of RealClimate or SkS admitting a message is wrong, their position is really one of the mucky old pot calling the smirror-finish electric kettle black.

    As for attributing the cooling of the stratosphere, you need to set out the reference from which the cooling is measured (and we become off-topic). Over decades, the CO2 cooling effect (or 'effects' - it is not a simple process) has been larger than the CFC/ozone cooling effects (McLandress et al 2014) but more recently it appears the reduced CFC/ozone cooling is apparently pretty-much matching the CO2 cooling (Ferraro et al 2015- [full text]).

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