According to a new paper in Oxford Open Climate Change, the strategies humanity must pursue to reduce climate change will have to include more than reducing greenhouse gases. This comes from an analysis of climate data led by researcher James Hansen.

Scientists have known since the 1800s that infrared-absorbing (greenhouse) gases warm Earth’s surface, and that the abundance of greenhouse gases changes naturally as well as from human actions. Roger Revelle, who was one of the early scientists to study global warming, wrote in 1965 that industrialization meant that human beings were conducting a “vast geophysical experiment” by burning fossil fuels, which adds carbon dioxide (CO₂) to the air. CO₂ has now reached levels that have not existed for millions of years.

Climate sensitivity

A long-standing issue concerns how much global temperature will rise for a specified CO₂ increase. A 1979 study released by the United States National Academy of Sciences concluded that doubling atmospheric CO₂ with ice sheets fixed would likely cause global warming between 1.5 and 4.5° Celsius. This was a large range, and there was additional uncertainty about the delay in warming caused by Earth’s massive ocean.

This new paper reevaluates climate sensitivity based on improved paleoclimate data, finding that climate is more sensitive than usually assumed. Their best estimate for doubled CO₂ is global warming of 4.8°C, significantly larger than the 3°C best estimate of the United Nations Intergovernmental Panel on Climate Change.

Aerosols

The authors also conclude that much of the expected greenhouse gas warming in the past century has been offset by the cooling effect of human-made aerosols—fine airborne particles. Aerosols have declined in amount since 2010 as a result of reduced air pollution in China and global restrictions on aerosol emissions from ships. This aerosol reduction is good for human health, as particulate air pollution kills several million people per year and adversely affects the health of many more people.

However, aerosol reduction is now beginning to unmask greenhouse gas warming that had been hidden by aerosol cooling. The authors have long termed the aerosol cooling a “Faustian bargain” because, as humanity eventually reduces air pollution, payment in the form of increased warming comes due.

This new paper predicts that a post-2010 acceleration of global warming will soon be apparent above the level of natural climate variability. The 1970-2010 global warming rate of 0.18°C per decade is predicted to increase to at least 0.27°C per decade during the few decades after 2010. As a result, the 1.5°C global warming level will be passed this decade and the 2°C level will be passed within the following two decades.

Policy

In a final section, Hansen describes his perspective based on decades of experience in trying to affect government policies. First, he believes that achievement of rapid phasedown of CO₂ emissions requires a rising domestic carbon fee with a border duty on products from nations without a carbon fee, as well as support of modern nuclear power to complement renewable energies.

Second, he argues that the West, which is primarily responsible for climate change, must cooperate with developing nations to help them achieve energy paths consistent with a propitious climate for all.