Warming in the pipeline: Decoding our climate commitment

Understanding Jim Hansen's new paper on climate commitment

Our planet is warming, and this is the result of emissions of carbon dioxide and other greenhouse gases that occurred over the last two centuries. The reason why emissions have a long-lasting effect is because carbon dioxide, once it's in the atmosphere, doesn't go away quickly. Instead, it hangs around for a really long time, continuing to warm the Earth throughout its stay.

This leads to one of the most policy-relevant scientific questions: if emissions stopped today, how much future warming will occur because of past emissions. This is sometimes referred to as the “zero emissions commitment” (abbreviated ZEC) or, more colloquially, as “warming in the pipeline.”

Over the last few years, it has become common wisdom that committed warming is zero, meaning that once emissions go to zero, the temperature will stop warming. This is expected because, after emissions cease, carbon dioxide is taken up by the ocean and land-biosphere, which reduces radiative forcing. But the ocean’s uptake of heat also declines, and these two factors basically cancel each other.

Results from ZEC experiments with 15 different climate models. Top: Change in energy flux from ocean warming (positive) and ocean and land CO2 uptake (negative) after net-zero emissions. Bottom: the amount of global mean surface temperature warming in each model after 50 years. Figure from Carbon Brief, adapted from adapted from Figure 7 in MacDougall et al (2020).

Global climate models simulate this, more or less, with model simulations showing that temperatures may rise or fall by about 0.3 degrees Celsius over the 50 years following the cessation of emissions.

Recently, Jim Hansen released a preprint of a paper that said:

Equilibrium global warming including slow feedbacks for today’s human-made greenhouse gas (GHG) climate forcing (4.1 W/m2) is 10°C, reduced to 8°C by today’s aerosols.

Since we have experienced about 1C of warming already, Hansen is saying that there is 7-9C of warming in the pipeline.

It turns out that the difference between the canonical “no warming in the pipeline” and Hansen’s 7-9C warming in the pipeline are different assumptions going into the calculations.

Difference #1: When people say there’s no warming in the pipeline, they are assuming that emissions go to zero. The carbon cycle keeps churning, though, and this causes atmospheric carbon dioxide to decline over the following decades.

Hansen, however, is assuming constant atmospheric concentration — the mixing ratio of carbon dioxide remains fixed in the atmosphere at today’s levels, around 420 ppm. This would require that global emissions of CO2 remain above zero for the next few millennia to compensate for the carbon that the carbon cycle removes.

Difference #2: When people say there’s no warming in the pipeline, they are typically looking at time periods shorter than a few centuries. On these time scales, the most important physics are processes that are often referred to as the “fast feedbacks”. These are processes that regulate atmospheric water vapor, the atmospheric lapse rate, clouds, and the parts of the cryosphere that react relatively quickly to the temperature of the climate, like sea ice.

On longer time scales, other processes become important. These “slow feedbacks” include phenomena such as carbon cycle feedbacks (changes in carbon sinks and sources as the planet warms) and ice-albedo feedback associated with the major ice sheets, Greenland and Antarctica, which (we think) will melt on millennial timescales.

So when Hansen says committed warming is 7-9C, he’s looking at very long time periods, millennia or beyond. His estimate, though, is higher than most other long-term “earth system sensitivity” values found in the literature.

Given these differing assumptions, both sides are right — if emissions stop today, we would get little warming over the coming decades, but we may get significant warming over thousands of years as slow feedbacks kick in. You can argue about how much warming we will eventually get (a good case can be made that it’s less than the 7-9C figure stated by Hansen), but his essential point that we would get substantially more warming in the long run with carbon dioxide concentrations at today’s levels is correct.

That said, Hansen’s assumptions will not happen. We are not going to keep atmospheric carbon dioxide and other greenhouse gases fixed for thousands of years. We will definitely stop burning fossil fuels in somewhere between a few decades to a few centuries and the concentration of greenhouse gases in our atmosphere will decline after that.

Considering that Jim Hansen's predictions have often proven correct, it's important that we pay close attention to what he’s saying. In this case, Hansen's perspective should be seen as a valuable addition to the usual understanding of the ‘warming in the pipeline.’ It encourages us to think about the long-term consequences that are often overlooked in discussions about climate policies that too often ignore anything happening beyond 2100.