Back in the 1960s, American oceanographer Roger Revelle already understood that burning fossil fuels was conducting a grand geophysical experiment by adding carbon dioxide (CO2) to the atmosphere, resulting in rapid global warming. Thus, if we want to limit this warming, it’s up to us to reduce our greenhouse gas emissions.
However, today, James Hansen, possibly the first person to make a significant declaration about anthropogenic climate change in 1988 before the U.S. Senate, suggests that this might not be enough. Two key factors are involved: climate sensitivity and aerosols.
A Question of Climate Sensitivity
Climate sensitivity to CO2 refers to the temperature increase associated with a doubling of greenhouse gas concentrations in our atmosphere. Sensitivity has long been a subject of debate. Drawing on more precise paleoclimatic data, James Hansen, now a researcher at Columbia University, argues in the Oxford Open Climate Change journal that our climate is more sensitive to CO2 than previously thought by climatologists.
While the Intergovernmental Panel on Climate Change (IPCC) estimated a value of +3°C (meaning a doubling of CO2 would lead to a global temperature increase of +3°C), Hansen’s work concludes a climate sensitivity of… +4.8°C!
Fewer Aerosols for Better Air Quality but Not for the Climate
The role of aerosols is the other crucial point that the Columbia University team emphasized. Throughout the 20th century, the high presence of aerosols in the atmosphere partly offset the effects of anthropogenic climate change. However, aerosols come with pollution and health risks. Since 2010, air pollution has been decreasing, leading to a reduction in atmospheric aerosols. As a result, temperatures have risen sharply.
This is what James Hansen is announcing. While the rate of climate warming was 0.18°C per decade between 1970 and 2010, it is expected to increase to at least 0.27°C per decade in 2010. This could push us past the 1.5°C warming threshold in this decade and even reach 2°C in the following two decades.
Reducing Emissions and Considering Geoengineering?
Ten years ago, James Hansen already estimated Earth’s energy imbalance—the surplus of energy absorbed from the Sun compared to thermal energy radiated back into space—at 0.6 watts per square meter (W/m2). To put it in perspective, he compared it to about 400,000 Hiroshima atomic bombs. Today, the imbalance is believed to be around 1.2 W/m2.
Given these conditions and on the eve of COP28, the researcher calls for the swift implementation of a progressively increasing national carbon tax with border adjustments for products from countries without a carbon tax. He also supports modern nuclear energy alongside renewable sources. Additionally, he suggests that the West, as a major contributor to climate change, should help developing countries find energy solutions compatible with the climate and beneficial for all.
Even with these measures in place, James Hansen believes that climate warming will reach dangerous levels. Therefore, he suggests researching temporary and targeted actions to address Earth’s significant energy imbalance. However, he acknowledges that such geoengineering actions, like stratospheric aerosol injection or saltwater cloud seeding, have unknown long-term and large-scale consequences, and many researchers are reluctant to consider them.