IRISH NEWS
Getting to net zero emissions by mid-century is conventionally understood as humanity’s best hope for keeping Earth’s surface temperature (already 1.2°C above its pre-industrial level) from increasing well beyond 1.5°C – potentially reaching a point at which it could cause widespread societal breakdown.
At least one prominent climate scientist, however, disagrees.
James Hansen of Columbia University in the US published a paper with colleagues in November which claims temperatures are set to rise further and faster than the predictions of the Intergovernmental Panel on Climate Change (IPCC). In his view, the 1.5°C target is dead.
He also claims net zero is no longer sufficient to prevent warming of more than 2°C. To regain some control over Earth’s rising temperature, Hansen supports accelerating the retirement of fossil fuels, greater cooperation between major polluters that accommodates the needs of the developing world and, controversially, intervening in Earth’s “radiation balance” (the difference between incoming and outgoing light and heat) to cool the planet’s surface.
There would probably be wide support for the first two prescriptions. But Hansen’s support for what amounts to the deliberate reduction of sunlight reaching Earth’s surface has brought into the open an idea that makes many uncomfortable.
Michael Mann from the University of Pennsylvania in the US and another titan of climate science, spoke for many when he dismissed solar radiation management as “potentially very dangerous” and a “desperate action” motivated by the “fallacy … that large-scale warming will be substantially greater than current-generation models project”.
Their positions are irreconcilable. So who is right – Hansen or Mann?
Earth’s radiation balance
First, an explanation.
There are only two ways to reduce global warming. One is to increase the amount of heat radiated from Earth’s surface that escapes to space. The other is to increase the amount of sunlight reflected back to space before it lands on something – whether a particle in the atmosphere or something on Earth’s surface – and is converted to heat.
There are many ways to do both. Anything that reduces the amount of greenhouse gas in the atmosphere will let more heat escape to space (replacing fossil fuels with renewables, eating less meat and tilling the soil less for example). Anything that makes the planet brighter will reflect more sunlight to space (such as refreezing the Arctic, making clouds whiter or putting more reflective particles in the atmosphere).
But the key difference between the two, in terms of their impact on global warming, is their response time. That is, the time it takes for a change in the factors that allow more heat to escape or sunlight to be reflected to appear as a change in Earth’s surface temperature.
Intervening to speed up the loss of heat from Earth’s surface cools the planet slowly, over decades and longer. Intervening to increase the sunlight Earth reflects back to space cools the planet more or less immediately.
The essence of the dispute between Mann and Hansen is whether reducing greenhouse gases, by a combination of reducing new emissions and permanently removing past emissions from the atmosphere, is now enough on its own to prevent warming from reaching levels that threaten economic and social stability.
Mann says it is. Hansen says that, while doing these things remains essential, it is no longer sufficient and we must also make Earth more reflective.
