▶ Watch Video: Bill Gates says reducing greenhouse emissions to zero is “bigger than anything humanity has done to date” It may come as a surprise, given the extensive body of evidence connecting humans to climate change, that directly-observed proof of the human impact on the climate had still eluded science. That is, until now. In a first-of-its-kind study, NASA has calculated the individual driving forces of recent climate change through direct satellite observations. And consistent with what climate models have shown for decades, greenhouse gases and suspended pollution particles in the atmosphere, called aerosols, from the burning of fossil fuels are responsible for the lion’s share of modern warming. In other words, NASA has proven what is driving climate change through direct observations — a gold standard in scientific research. “I think most people would be surprised that we hadn’t yet closed this little gap in our long list of evidence supporting anthropogenic [human-caused] climate change,” says Brian Soden, co-author of the study and professor of Atmospheric Sciences at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science. By now it’s common knowledge that the rapid warming of the past century is not natural. Rather, it is a result of the build up of heat-trapping greenhouse gases like carbon dioxide (CO2) and methane, much of it from the burning of fossil fuels. 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 trapped by specific molecules like CO2, methane and water vapor. Simply, the more greenhouse gases the atmosphere has, the more heat is trapped and the more the temperature goes up. This NASA animation is a simplified illustration of Earth’s planetary energy balance: The energy budget is balanced between incoming (yellow) and outgoing radiation (red). Natural and human-caused processes affect the amount of energy received as well as the amount emitted back to space. NASA Since the mid 1800s, CO2 in the atmosphere has increased from 280 parts per million to 415 parts per million — a 50% increase — and it is now the highest it has been in at least 3 million years. Carbon dioxide in the atmosphere is increasing at a pace 100 times faster than it naturally should. At the same time, suspended pollution particles, called aerosols, cool the atmosphere by blocking sunlight. This unintentional side effect of the Industrial Revolution has proven useful in masking some greenhouse warming. While these particles were effective at helping counteract some of the global warming in the mid to late 20th century, their impact is diminishing, because since the 1980s pollution has been gradually clearing up. While this is great news for health, it is unmasking additional warming in the system. Together, the change in heat absorbed in our atmosphere because of changes in greenhouse gases and aerosols is called “radiative forcing.” These changes in radiative forcing throw off Earth’s energy balance. That’s because, in order for Earth’s average temperatures to remain steady, the “energy-in” from the sun must be equalized by the “energy-out” from Earth into space. When those numbers are equal the Earth maintains balance. But when greenhouse gases build up, the energy going out is less than the energy entering the Earth system, which heats up 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 from specialized satellite observations to determine how much each component warms or cools the atmosphere. To no one’s surprise, what they have found is that the radiative forces, which computer models have indicated for decades were warming the Earth, match 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 indirect evidence of the factors warming the Earth. The predicted energy imbalance illustrated by decades’ worth of computer models has become apparent for all of humanity to see, from disappearing glaciers to more extreme weather disasters to warming oceans. “We have long had good evidence that the predicted energy imbalance was real because of the increases in ocean heat content. That is very powerful confirmation that the models were predicting warming for the right reasons,” Schmidt explains. 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 comprehensive evaluation. Soden adds that science does have solid 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 theoretically should cause the planet to warm at roughly the rate observed over the last century. However, Soden says that observing the trapping of heat from space is actually quite challenging. This new research solves that challenge. “This is the first calculation of the total radiative forcing of Earth using global observations, accounting for the effects of aerosols and greenhouse gases,” said Ryan Kramer, first author on the paper and a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s direct evidence that human activities are causing changes to Earth’s energy budget.” Specifically, this study has been able to calculate solid numbers for the changes in heat trapped in the Earth system from the individual contributors that influence heat transfer, like radiation, clouds and water vapor, for the period 2003-2019. The researchers did that by analyzing satellite observations and applying what they call “radiative kernels” to disentangle the various components controlling the transfer, absorption and emission of heat inside the Earth system and what is sent back out into 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 which account for a smaller but still important amount of warming. One 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, further allowing for more water vapor to build up. This is a positive feedback which perpetuates warming. The result: From 2003 through 2018, radiative forcing has increased 0.5 watts per meter-squared (W/m2), which accounts for the planetary imbalance, the excess heat trapped in the Earth system. The researchers conclude that this increase has indeed been due to a combination of mainly rising concentrations of greenhouse gases and, to a lesser degree, recent reductions in aerosol emissions. For reference, Schmidt says the excess .5 W/m2 added to the Earth system from 2003-2018 is roughly equivalent to one Christmas tree lightbulb for every 5-foot-square area on Earth. That may not sound like very much, but that much energy would be expected to warm the planet by more than a half a degree Fahrenheit in only 16 years. To put it another way, the 0.5 W/m2 of excess heat absorbed by the Earth system is 10 times the total energy used by humans in a year, meaning everything from cooking stoves to nuclear power. “In reality, the observational results came in just as predicted by the theory,” says Soden. “There is no surprise in the results, but rather it’s really more of ‘dotting the i’s and crossing the t’s’ on anthropogenic [human-caused] climate change. It closes that last link between rising CO2 levels and planetary warming.” But this study does more than just provide concrete evidence of the link between humans and recent climate change. It also illustrates just how far science has come in uncovering the secrets which govern the workings of our physical universe.