Quantcast
Popular
An annular solar eclipse in progress graces the sky at Dog Beach in San Diego, California, USA, 1992. N J Gargasz / Alamy

Solar Eclipse: Why the Sun Is Not Responsible for Recent Climate Change

By Zeke Hausfather

With a total solar eclipse sweeping across North America, everyone is suddenly paying attention to the sun. One of the most common skeptical 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 meter 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, told Carbon Brief that it would take around a seven watts per meter 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, said 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, said 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 told 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 discernible 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, said Dr. Mike Lockwood, a solar physicist at the University of Reading. As he concluded 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."

Reposted with permission from our media associate Carbon Brief.

Show Comments ()

EcoWatch Daily Newsletter

Sponsored
Business
velkr0 / Flickr / CC BY 2.0

Texas Supreme Court Rules Cities Cannot Ban Plastic Bags

The Texas Supreme Court struck down the city of Laredo's plastic bag ban—a decision that will likely overturn similar bans in about a dozen other cities, including Austin, Fort Stockton and Port Aransas.

Keep reading... Show less
Politics
Ryan Zinke visits Wall Drug in Wall, South Dakota on May 25. Sherman Hogue / U.S. Dept. of the Interior

Report: Trump Admin. Suppressing Media Access of Government Scientists

A new Trump administration protocol requires U.S. Geological Survey (USGS) scientists to run interview requests with the Department of the Interior, its parent agency, before speaking to journalists, the Los Angeles Times reported.

The move is a departure from past media practices that allowed government scientists to quickly respond to journalists' inquiries, according to unnamed USGS employees interviewed by the Times.

Keep reading... Show less
Climate
Icebergs calving from an ice shelf in West Antarctica. NASA / GSFC / Jefferson Beck / CC BY-SA 2.0

Good News From Antarctica: Rising Bedrock Could Save Vulnerable Ice Sheet

After last week's disturbing news that ice melt in Antarctica has tripled in the last five years, another study published Thursday offers some surprising good news for the South Pole and its vulnerable West Antarctic Ice Sheet (WAIS).

The study, published in Science by an international research team, found that the bedrock below the WAIS is rising, a process known as "uplift," at record rates as melting ice removes weight, potentially stabilizing the ice sheet that scientists feared would be lost to climate change.

Keep reading... Show less
GMO
Soybeans with cupped leaves, a symptom of dicamba injury. University of Arkansas System Division of Agriculture / Flickr / CC BY-NC 2.0

Dicamba Damage Roars Back for Third Season in a Row

University weed scientists have reported roughly 383,000 acres of soybean injured by a weedkiller called dicamba so far in 2018, according to University of Missouri plant sciences professor, Kevin Bradley.

Dicamba destroys mostly everything in its path except the crops that are genetically engineered (GE) to resist it. The drift-prone chemical can be picked up by the wind and land on neighboring non-target fields. Plants exposed to the chemical are left wrinkled, cupped or stunted in growth.

Keep reading... Show less
Sponsored
Food
Memphis Meats

FDA Takes First Steps to Regulating Lab-Grown Meat

By Dan Nosowitz

Lab-grown meat—also known as cultured meat or in vitro meat—has long been enticing for its potential environmental, social and economic benefits.

Keep reading... Show less
Politics
Scott Pruitt speaking at meeting at the USDA headquarters in Washington, DC, on Jan. 17. Lance Cheung / USDA

Breaking: Sierra Club Demands Pruitt’s Emails After Only 1 Disclosed by EPA

As part of ongoing litigation, the Sierra Club has demanded that the U.S. Environmental Protection Agency (EPA) search Scott Pruitt's personal email accounts for work-related emails, or certify clearly and definitively that the administrator has never used personal email for work purposes. The demand comes on the heels of a successfully litigated Freedom of Information Act (FOIA) request for all of EPA Administrator Scott Pruitt's email and other communications with all persons and parties outside the executive branch. These facts were first reported in Politico early this morning.

Keep reading... Show less
Sponsored
Animals

Iceland Flouts Global Ban to Slaughter First Protected Fin Whale of New Hunting Season

Iceland's multi-millionaire rogue whaler Kristján Loftsson and his company Hvalur hf have resumed their slaughter of endangered fin whales in blunt defiance of the international ban on commercial whaling.

The hunt is Iceland's first in three years and marks the start of a whaling season that could see as many as 239 of these majestic creatures killed.

Keep reading... Show less
Food
Life- Trac / CC BY-SA 3.0

Farm Bill With Huge Giveaways to Pesticide Industry Passes House

A farm bill that opponents say would harm endangered species, land conservation efforts, small-scale farmers and food-stamp recipients passed the U.S. House of Representatives 213 to 211, with every House Democrat and 20 Republicans voting against it, The Center for Biological Diversity reported.

Keep reading... Show less
Sponsored

mail-copy

The best of EcoWatch, right in your inbox. Sign up for our email newsletter!