It may come as a surprise, given the extensive body of evidence people are associated with climate change, that directly observed evidence of human impact on climate still eluded science. That is, until now.
In a first study of its kind, NASA calculated the individual drivers of recent climate change through direct satellite observations. And in line with what climate models have shown for decades, greenhouse gases and airborne pollutants in the atmosphere, called aerosols, from fossil fuel combustion are responsible for the lion’s share of modern warming.
In other words, NASA has proven through direct observations what causes climate change – a gold standard in scientific research.
“I think most people would be surprised that we hadn’t yet closed this tiny hole in our long list of evidence supporting anthropogenic [human-caused] climate change, ”said Brian Soden, study co-author and professor of Atmospheric Sciences at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.
It is now common knowledge that the rapid heating of the past century of course notRather, it is the result of the build-up of greenhouse gases that trap heat, such as carbon dioxide (CO2) and methane, much of which comes from the fossil fuel combustion
The science behind why the Earth is warming
When sunlight enters the atmosphere, some of it is reflected back to space without heating the Earth. The rest is absorbed by the Earth’s surface and atmosphere and re-radiated as heat. Some of this heat escapes back into space, but the rest of the heat is retained by specific molecules such as CO2, methane and water vapor. Simply put, the more greenhouse gases the atmosphere has, the more heat is retained and the more the temperature rises.
NASA
Since the mid-19th century CO2 in the atmosphere has increased from 280 parts per million to 415 parts per million – an increase of 50% – and it is now the highest it has been in at least 3 million years. Carbon dioxide in the atmosphere is increasing 100 times faster than it normally should.
At the same time, suspended pollution particles called aerosols cool the atmosphere by blocking sunlight. This unintended side effect of the industrial revolution has proved useful in masking greenhouse warming.
While these particles were effective in counteracting some of the global warming in the mid to late 20th century, their impact is diminishing as pollution has been gradually clearing up since the 1980s. While this is great news for health, it exposes additional heating in the system.
Together, the change in heat absorbed into our atmosphere as a result of changes in greenhouse gases and aerosols is called “radiative forcing.” These changes in radiative forcing throw off the Earth’s energy balance. That’s because, in order to keep the average temperatures on Earth stable, the “energy-in” of the sun must be equalized by the “energy-out” of the Earth into space.
When those numbers are equal, the Earth maintains balance. But when greenhouse gases accumulate, the outgoing energy is less than the energy entering the Earth system, heating our oceans and atmosphere, creating an imbalance in the Earth’s energy budget.
What NASA has done in this study is to calculate or quantify the individual forcings measured with specialized satellite observations to determine how much each part is heating or cooling the atmosphere. Unsurprisingly, they found that the radiative forces indicated by computer models for decades warming the Earth matched the changes they measure in observations.
New insight from NASA
Gavin Schmidt, the director of NASA Goddard Institute for Space Studies, says science has long had an overwhelming amount of circumstantial evidence of the factors that are warming the Earth. The predicted energy imbalance, exemplified by decades of computer modeling, has become apparent to all of humanity disappearing glaciers to more extreme weather conditions to warming oceans
“We have long had good evidence that the predicted energy imbalance was real because of the increase in the ocean’s heat content. That is 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 affected the transfer and absorption of heat into the atmosphere, but only in localized environments, not a comprehensive evaluation.
Soden adds that science has solid observational evidence that CO2 has increased over the past century due to the combustion of greenhouse gases and that laboratory measurements confirm that CO2 is absorbing heat, which theoretically should allow the planet to warm at about the rate it has seen over the past century. has been observed. century. However, Soden says observing heat retention from space is actually quite a challenge. This new research solves that challenge.
“This is the first calculation of Earth’s total radiative forcing using global observations that takes into account the effects of aerosols and greenhouse gases,” said Ryan Kramer, lead author of the paper and researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. . “It is direct evidence that human activities are causing changes in the Earth’s energy budget.”
Specifically, for the period 2003-2019, this study has been able to calculate solid numbers for the changes in heat retained in the Earth system of the individual contributors that influence heat transfer, such as radiation, clouds and water vapor. The researchers did this by analyzing satellite observations and using what they call “radiation grains” to disentangle the various components that control the transfer, absorption and emission of heat in the Earth’s system and what is sent back into space. Until now, satellite observations of the Earth’s radiation budget had only measured the sum of the radiation changes, not the individual components.
Then there are feedbacks in the climate system that are responsible for a smaller but still significant amount of warming. An example of this is the fact that as the atmosphere heats up, it can hold more water vapor, and that means it can hold more heat, allowing more water vapor to build up. This is a positive feedback that maintains warming.
The result: From 2003 to 2018, radiative forcing has increased by 0.5 watts per square meter (W / m2), which is responsible for the planetary imbalance, the excess heat trapped in the Earth’s system. The researchers conclude that this increase is indeed the result of a combination of mainly rising concentrations of greenhouse gases and, to a lesser extent, recent reductions in aerosol emissions.
For reference, Schmidt says the excess 0.5 W / m2 added to the grounding system between 2003 and 2018 is roughly equivalent to one Christmas tree lamp for every 1.5 square meter area on Earth. That may not sound like much, but that much energy was expected to warm the planet by more than half a degree Fahrenheit in just 16 years. In other words, the 0.5 W / m2 excess heat absorbed by the grounding system is 10 times the total energy that humans use in a year, so everything from stoves to nuclear power.
“In reality, the observation results came in as predicted by the theory,” says Soden. “The results are not surprising, but rather it is ‘dots on the i and crosses’ on anthropogenic [human-caused] climate change. It closes that last link between rising CO2 levels and global warming. “
But this study offers more than just concrete evidence of the link between humans and recent climate change. It also illustrates how far science has come in revealing the secrets that govern the workings of our physical universe.