NASA measures direct evidence that humans are causing climate change

It may come as a surprise, given the extensive body of evidence connecting humans to climate change, that directly observed evidence of human impact on climate had still eluded science. That is, until now.

In a one-of-a-kind study, NASA has calculated the individual driving forces of recent climate change through direct satellite observations. And according to what climate models have shown for decades, greenhouse gases and airborne pollutants, called aerosols, from the burning of fossil fuels are responsible for most of modern warming.

In other words, NASA has shown what is driving climate change through direct observations, a gold standard in scientific research.

“I think most people would be surprised that we have not yet closed this small gap in our long list of evidence supporting [human-caused] climate change, “says Brian Soden, co-author of the study and professor of Atmospheric Sciences at the Rosenstiel School of Marine and Atmospheric Sciences at the University of Miami.

By now it is common knowledge that the rapid heating last century is it’s not natural. Rather, it is the result of the accumulation of heat-trapping greenhouse gases, such as carbon dioxide (CO2) and methane, in much of the Burning of fossil fuels.

The science behind why the Earth is warming

When sunlight enters the atmosphere, some of it is reflected back into space without heating the Earth. The rest is absorbed by the Earth’s surface and atmosphere and is re-radiated as heat. Some of this heat escapes into space, but the rest of the heat is trapped by specific molecules such as CO2, methane, and water vapor. Quite simply, the more greenhouse gases in the atmosphere, the more heat is trapped and the more the temperature rises.

This NASA animation is a simplified illustration of Earth’s planetary energy balance: the energy balance is balanced between incoming radiation (yellow) and outgoing radiation (red). Natural and man-made processes affect the amount of energy received, as well as the amount emitted back into space.


Since the middle of the 19th century, CO2 in the atmosphere it has risen from 280 parts per million to 415 parts per million, a 50% increase, and is now the highest in at least 3 million years. Carbon dioxide in the atmosphere is increasing at a rate 100 times faster than it naturally should.

At the same time, pollutant particles in suspension, called aerosols, cool the atmosphere by blocking sunlight. This unintended side effect of the Industrial Revolution has proven helpful in masking some of the greenhouse warming.

While these particles were effective in helping to counteract some of the global warming of the mid to late 20th century, their impact is diminishing, because since the 1980s pollution has gradually cleared up. While this is great health news, it is unmasking additional heating in the system.

Collectively, the change in heat absorbed into our atmosphere due to changes in greenhouse gases and aerosols is called “radiative forcing.” These changes in radiative forcing alter the energy balance of the Earth. This is because, for the Earth’s average temperatures to remain stable, the “energy input” from the sun must be matched by the “energy output” from the Earth to space.

When those numbers are equal, the Earth maintains equilibrium. But when greenhouse gases accumulate, the energy that goes out is less than the energy that enters the Earth’s system, warming our oceans and our atmosphere, creating an imbalance in Earth’s energy budget.

What NASA has done in this study is calculate or quantify the individual forcings measured from specialized satellite observations to determine how much each component heats or cools the atmosphere. To no one’s surprise, what they have found is that radiative forces, which computer models have indicated for decades were warming the Earth, match the changes they measure in observations.

New information from NASA

Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, says science has long had an overwhelming amount of indirect evidence of factors that warm the Earth. The predicted energy imbalance illustrated by decades of computer modeling has become apparent to all of humanity, since disappearing glaciers More extreme weather disasters for the warming of the oceans.

“We’ve had good evidence for a long time that the predicted energy imbalance was real due to the increased heat content of the ocean. That’s a very powerful confirmation that the models predicted warming for the right reasons,” explains Schmidt. He says scientists have also had direct evidence that changes in greenhouse gases have been affecting the transfer and absorption of heat in the atmosphere, but only in localized settings, not a full assessment.

Soden adds that science has strong observational evidence that CO2 has increased over the last century due to the burning of greenhouse gases and that laboratory measurements confirm that CO2 absorbs heat, which should theoretically make the planet warm at roughly the rate observed in the last century. However, Soden says that observing heat capture from space is actually quite challenging. This new research solves that challenge.

“This is the first calculation of the Earth’s total radiative forcing using global observations, taking into account the effects of aerosols and greenhouse gases,” said Ryan Kramer, first author of the paper and researcher at the Goddard Space Flight Center of NASA in Greenbelt, Maryland. . “It is direct evidence that human activities are causing changes in the Earth’s energy budget.”

Specifically, this study has been able to calculate solid numbers for changes in heat trapped in the Earth’s system from individual contributors influencing heat transfer, such as radiation, clouds, and water vapor, for the period 2003. -2019. The researchers did this by analyzing satellite observations and applying what they call “radiative nuclei” to untangle the various components that control the transfer, absorption and emission of heat within the Earth system and what is sent back to space. Up to this point, satellite observations of Earth’s radiation budget had only measured the sum total of radiation changes, not the individual components.

Then there are also feedbacks in the climate system that explain a smaller but still significant amount of warming. An example of this is the fact that as the atmosphere warms, it can hold more water vapor, and that means it can trap more heat, allowing more water vapor to accumulate. This is positive feedback that perpetuates the warm-up.

The result: From 2003 to 2018, radiative forcing has increased by 0.5 watts per square meter (W / m2), which explains the planetary imbalance, the excess heat trapped in the Earth’s system. The researchers conclude that this increase has been due to a combination of mainly increasing concentrations of greenhouse gases and, to a lesser extent, recent reductions in aerosol emissions.

For reference, Schmidt says that the excess .5 W / m2 added to the Earth’s system between 2003 and 2018 is roughly equivalent to one Christmas tree light bulb for every 5 square feet area on Earth. It may not sound like much, but that much energy would be expected to warm the planet by more than half a degree Fahrenheit in just 16 years. To put it another way, the 0.5 W / m2 of excess heat absorbed by Earth’s system is 10 times the total energy used by humans in a year – that is, everything from cooking stoves to nuclear power. .

“Actually, the results of the observation were obtained just as the theory predicted,” says Soden. “There is no surprise in the results, but it is actually more about ‘dotting the i’s and crossing the t’ in anthropogenic [human-caused] climate change. It closes the last link between rising CO2 levels and global warming. “

But this study does more than just provide concrete evidence of the link between humans and recent climate change. It also illustrates how far science has come in uncovering the secrets that govern the workings of our physical universe.


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