As warming crosses one degree Celsius, Hansen and his colleagues’ research shows that additional heat is stored mostly in the deep ocean, where it can remain locked away for hundreds or thousands of years. Water circulates very slowly down there. That essentially locks in further climate change, even if emissions are drastically reduced later on, because that circulating water will continually replenish the surface with relative warmth from below. Additional warming will also begin to trigger feedbacks (melting permafrost, thawing methane) that will unleash additional greenhouse gases and drive further warming.
As warming approaches two degrees, it locks in an additional 10-20 meters of sea level rise over the next few hundred years—enough to flood every coastal city in the world. Ecosystem collapse would be virtually assured, as plants and animals that have evolved into precise niches over hundreds of thousands of years are forced to adapt to new conditions in just a decade or two. Even assuming we eventually stop emitting CO2 completely, reaching two degrees could, the study shows, mean we remain above one degree for hundreds of years or more.
And if warming goes over two degrees, Hansen and his colleagues present a familiar litany of climate impacts: mass extinctions, stronger storms, and increasingly severe effects for human health, along with “major dislocations for civilization.”
The study’s key takeaway is that unless CO2 emissions peak right about now — which they are clearly not doing — in just a few more years we will lock in a two degrees rather than a one degree temperature rise. That will set climate impacts in motion for the next thousand years or so, barring advances in technology that are currently largely discredited as either too expensive or too impractical on the scale necessary to reverse the warming that’s already baked into the system.