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.
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.
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.
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.
Could mouthwash help stop the spread of the new coronavirus?
- How to Stop Touching Your Face to Minimize Spread of Coronavirus ... ›
- Vodka Won't Protect You From Coronavirus, and 4 Other Things to ... ›
EcoWatch Daily Newsletter
By Naomi Larsson
For centuries, the delicate silver dove has been a symbol of love and fidelity.
Biodiversity and Habitat Loss<p>Their near extinction is a symbol of the <a href="https://www.dw.com/en/global-biodiversity-outlook-targets-extinction-summit-new-york-pledge/a-54932895" target="_blank">biodiversity crisis</a> in the UK, largely driven by habitat destruction. Britain is now one of the countries with the most <a href="https://www.wwf.org.uk/future-of-UK-nature#:~:text=The%20UK%20is%20one%20of,than%20half%20are%20in%20decline" target="_blank">depleted nature</a> in the world according to the World Wildlife Fund. Half its plant and animal species are in decline and more than <a href="https://www.rspb.org.uk/about-the-rspb/about-us/media-centre/press-releases/let-nature-sing-wales/#:~:text=a%20natural%20tragedy.-,Over%2040%20million%20birds%20have%20vanished%20from%20UK%20skies%20in,unaware%20of%20the%20impending%20danger" target="_blank">40 million birds</a> have vanished in just half a century.</p><p>"[Turtle doves] are the canary in the [coal] mine because there are all these other species before it and after it," said Tree. "It's an umbrella for all the other species that are heading that way."</p><p>Turtle doves migrate south through Europe to sub-Saharan Africa between July and September, ending up in dry woodland and farmland areas of countries like Mali and Senegal for winter. </p><p>Droughts in West Africa and the Sahel region are believed to have contributed to the fall in turtle dove species recorded in northern Europe, with low rainfall reducing supplies of the seeds and insects the birds rely on for energy for the long journey home.</p>
Conservation and Farming<p><a href="https://www.operationturtledove.org/" target="_blank" rel="noopener noreferrer">Operation Turtle Dove,</a> a partnership project of charities including the Essex Wildlife trust, works with landowners and farmers to actively build turtle dove habitat.</p><p>Outten works with <a href="https://www.ebws.org.uk/birdsites/blue-house-farm-ewt-north-fambridge" target="_blank">Blue House Farm</a>, a 660-acre nature reserve in the UK county of Essex, where they have replicated weedy fallow plots. </p><p>"We work on it every year to make sure it's in the condition it needs to be with plants such as clovers and black medic," Outten said. "These plants are native to the landscape and produce the seed the birds feed on." </p><p>The birds eat a wide range of seeds from various plants that would have been abundant 50 or 100 years ago, added Guy Anderson, program manager for species recovery with The Royal Society for the Protection of Birds (RSPB). </p><p>"But it's simply true that with the gradual process of <a href="https://www.dw.com/en/farming-without-pesticides-how-can-we-make-agriculture-greener/a-52216796" target="_blank" rel="noopener noreferrer">intensifying our agricultural production</a>, the availability of those seeds has dropped and dropped," said Anderson.</p><p>Part of the project includes supplementary feeding — providing sources of food in the form of seed or grain. Under the Countryside Stewardship Scheme in England, farmers can receive financial support to create a turtle dove habitat. </p><p>Though they haven't recorded an increase in doves across the sites in the four years of working on the project, Outten said they are seeing improvements in how landowners and farmers manage habitat for the birds. </p>
A Turtle Dove Haven<p>The 3,500-acre Knepp Estate in West Sussex is another project taking a different approach and one of the few places where turtle dove numbers are increasing.</p><p>Isabella Tree and her husband Charlie Burrell converted their intensively farmed land into a rewilding project almost 20 years ago. They have let the land return to nature.</p><p>Just one year after they'd finished <a href="https://www.dw.com/en/uks-most-talented-architects-are-not-human/a-35952128" target="_blank">rewilding</a> the southern part of their property, they heard turtle doves for the first time. It's now a breeding hotspot for the birds with an estimated 19 pairs. Knepp is also home to <a href="https://www.rewildingbritain.org.uk/rewilding/rewilding-projects/knepp-estate" target="_blank" rel="noopener noreferrer">2% of the UK's population</a> of nightingales. </p><p>Tree is critical of supplementary feeding schemes that, in her view, are short term. She questions the chances of turtle doves getting to feed on scattered seeds before other mammals eat them first.</p>
- 41% of UK Species Have Declined Since 1970, Major Report Finds ... ›
- One in Eight Bird Species Threatened With Extinction, Study Finds ... ›
- Pesticides to Blame for UK's Declining Turtle Dove Population ... ›
We pet owners know how much you love your pooch. It's your best friend. It gives you pure happiness and comfort when you're together. But there are times that dogs can be very challenging, especially if they are suffering from a certain ailment. As a dog owner, all you want to do is ease whatever pain or discomfort your best friend is feeling.
Life-sized, ultra-realistic robotic dolphins could help end animal captivity by replacing living creatures in aquariums and theme parks.
- Keeping Large Mammals Captive Damages Their Brains - EcoWatch ›
- Scientists Combine AI With Biology to Create Xenobots, the World's ... ›
- Singapore Uses 'Scary' Robot Dog to Enforce Social Distancing ... ›
By Jessica Corbett
Green groups applauded Sen. Jeff Merkley on Wednesday for introducing a pioneering pair of bills that aim to "protect the long-term health and well-being of the American people and their economy from the catastrophic effects of climate chaos" by preventing banks and international financial institutions from financing fossil fuels.