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Vucea OP t1_jdx4blf wrote

Once we emit about 1000 gigatons of carbon, much of the massive ice sheet will melt irreversibly. We’ve emitted 500 gigatons so far.

The Greenland Ice Sheet covers 1.7 million square kilometers (660,200 square miles) in the Arctic. If it melts entirely, global sea level would rise about 7 meters (23 feet), but scientists aren’t sure how quickly the ice sheet could melt. Modeling tipping points, which are critical thresholds where a system behavior irreversibly changes, helps researchers find out when that melt might occur.

Based in part on carbon emissions, a new study using simulations identified two tipping points for the Greenland Ice Sheet: releasing 1000 gigatons of carbon into the atmosphere will cause the southern portion of the ice sheet to melt; about 2500 gigatons of carbon means permanent loss of nearly the entire ice sheet.

Having emitted about 500 gigatons of carbon, we’re about halfway to the first tipping point.


grundar t1_jdxicxh wrote

> Once we emit about 1000 gigatons of carbon, much of the massive ice sheet will melt irreversibly.

That is not an accurate summary of the paper.

From "Discussion":
> "We find two critical temperature anomaly thresholds above which the equilibrium volume of the GIS decreases non-linearly, at approximately 0.6 and 1.6°C. However, a temporary overshoot of these critical temperature thresholds does not inevitably cause long-term melt of the ice sheet (see Section 3.2). With transient experiments, we find that an equilibrium state of the GIS with smaller ice volume is approached only if the CO2 forcing is applied sufficiently long that the ice volume falls below the values of ΔVA = −0.26 m sle and ΔVC = −2.4 m sle, respectively."

The paper examines this in detail in Section 3.2; in particular, they say:
> "a temporal overshoot of a critical temperature does not necessarily lead to long-term ice loss"

So what does that mean?

What that means is if we reduce our emissions quickly enough, then net CO2 sequestration (from natural sources like silicate weathering as well as possibly from manmade sources such as direct air capture) will reduce the level of CO2 in the atmosphere, and hence the temperature, and hence potentially pull us back to the other side of the temperature tipping points before the ice loss tipping point is reached.

So what does that mean?

It means that even "tipping points" operate on geologic time scales, meaning they are generally not irreversible on human time scales. Passing one of the critical temperatures in the paper (1.6C) for 2,000 years would bring irreversible melt; passing it for 20 years would not. As a result, speed of decarbonization matters, regardless of where we are with regard to different tipping points.