Because it’s in the news, this is a thread on #HothouseEarth.
@bobkopp has one too
But I want to say some other things. First bit a summary, then my thoughts.
@bobkopp has one too
But I want to say some other things. First bit a summary, then my thoughts.
This cartoon in their paper is an anchor point for the thread, which the authors (Steffen et al.) present to frame their discussion of climate stability and tipping points 
Stefan et al. envision Earth’s climate as resting in a stable state in the various valleys in the cartoon, or at least oscillate between but remain bounded by two states (e.g., glacial-interglacial limit cycles).
The G-IG cycles essentially characterize Earth’s climate over the last million-ish years, following a longer-term decline in atmospheric CO2 over the last several tens of millions of years and the formation of ice sheets.
Give sufficient warming, they argue Earth could reach a very stable “hothouse Earth” state, one that is roughly irreversible where “biogeophysical feedbacks in the Earth System..could become the dominant processes controlling the system’s trajectory”
In such a regime, they suggest these feedbacks will “rais[e] the temperature further to activate other tipping elements in a domino-like cascade that could take the Earth System to even higher temperatures”
They identify four time periods that capture the essence of where Earth is near but moving away from or moving toward depending on time and emission trajectory. The mid-Holocene, Eemian interglacial, mid-Pliocene, and mid-Miocene.
The mid-Holocene was ~6,000-7,000 years ago, roughly comparable global mean temperature and lower CO2 (somewhat different orbital state). They argue we are moving away from this state and that it is not accessible in stability-space.
Eemian was the last major interglacial in the limit cycles I mentioned, ~125,000 years ago, ~ a degree warmer than modern, lower CO2, sea level many meters higher & again different orbital state favoring warmer poles. As before, they suggest we are moving away from this state…
…as CO2 is still rising rapidly, and we given emission trajectories we should equilibrate to a warmer climate.
The mid-Pliocene (~3 million yrs ago) is the last geologic time with comparable CO2 levels and probably 2-3 degrees warmer than pre-industrial. They suggest we are rapidly approaching this state, and is actually a “best case scenario”
The mid-Miocene (15-17 million years ago) with global temps 4-5 C warmer than pre-industrial and sea levels tens of meters higher, is their vision of a “hothouse” that we are likely to relax into given high emissions.
They could have picked other intervals as well (Eocene, Cretaceous, etc.) but I think the framing here is basically correct. We're moving away from something like the mid-Holocene/Eemian (which had different forcings), closing in on the Pliocene, worst case could be warmer.
What precisely a “hothouse” is isn’t defined well, but in climate circles, we typically imagine a climate a couple degrees warmer at the equator & several times that near the poles (i.e., a shallower pole-to-equator T gradient), perhaps to the point of ice-free polar conditions.
The feedbacks they talk about in relation to such a transition are mixed variety. In our field, it is useful to distinguish between feedbacks for a given CO2 (and temp) change of the fast flavor (e.g., water vapor, sea ice, clouds) & slow (ice sheet and vegetation distribution).
Then also feedbacks that affect the GHG concentration (on top of what humans do directly), like declining efficiency of carbon sinks in the biosphere & ocean, amazon forest dying, permafrost thawing, etc. There’s various ways to do a feedback accounting. Just don’t double count.
Anyway, time for some opinions. As @bobkopp said, there’s nothing new in here. But there might be some differences in how I conceptualize this stuff vs. the author’s or the media interpretations, or maybe not.
First, I still maintain it is much more useful to think of climate change as a linear problem- one in which temperature grows monotonically and where the maximum temperature encountered increases with cumulative carbon emitted.
One does not need to invoke tipping points to make this a big problem. A gradual warming can still get bad if it continues. But, also, it makes for clumsy philosophy when we’ve crossed some threshold (it’s self-perpetuating now, too late, let’s keep burning fossil fuels!)
As for stability, it’s not clear to me a hothouse is as irreversible as the deep valley in their figure suggests. We’ve exited hothouses in the past based on a smooth decline in CO2. There’s a lot of wiggle room for the climate to move around just based on what CO2 does.
Of course, there are some sub-systems (e.g. Amazon rainforest or ice sheet) that don’t just re-appear if they go away & you then go back to pre-industrial CO2. Sea ice and temperature are pretty reversible, though.
A lot of the big, slow ones are on that multi-century timescale
A lot of the big, slow ones are on that multi-century timescale
While these are potentially a big deal on that long timescale, I’ve never been a fan of “planetary thresholds” and points of no return framing. Scientifically, they are dicey concepts, and politically, haven’t been leveraged in ways I suspect people want them to be.
The authors didn’t do this, but some coverage did— I always have to throw in here that “runaway warming” is certainly not the correct framing. Warming does not runaway in all except very extreme planetary circumstances, of which Earth is not at much risk.
But the take-home points from the article concerning the vulnerability of agricultural systems, more disparity between wealthy and poor countries, and general disruption to human societies is important to keep in mind.
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