Virtually everyone says that the one of the magic numbers of our environment, the upper limit for temperature rise to avoid unacceptable human impacts is (everyone say it with me) 2°C above pre-industrial levels. As I pointed out back in June (Two degrees of separation), this very critical number has an interesting, to say the least, background.
In that prior posting, I said that the only sources I could find on the matter traced it to the late 1980s. Now there’s a post on Green Car Congress that pushes the origin of that number back to a Stockholm meeting in 1972. While I highly recommend the whole GCC piece, Devil In The Details: Is Copenhagens 2°C Guardrail Obsolete?, here’s a taste:
While the IPCC had acknowledged by 2001 that historical greenhouse gas emissions had likely already committed some ecosystems to irreversible changes, the predominant view was that a 2°C warming limit would limit such changes to acceptable levels in most parts of the world, and that the collapse of large-scale ecosystems, now known as tipping elements, would be unlikely unless warming rose by 3 to 4 degrees.
However, a literature review of recent climate research, as well as the most recent IPCC climate assessment report (2007 Fourth Assessment Report, or AR4), indicates that at least some of the rationale for a 2°C guardrail, or warming limit, as well as a peak atmospheric concentration limit of 450 ppm CO2, has failed to take into account key drivers of climate change, resulting in an underestimation of its potential effects at a given temperature.
The AR4 Synthesis Report acknowledged in 2007, for example, that “based on current understanding of climate-carbon cycle feedbacks, model studies suggest that stabilizing CO2 concentrations at, for example, 450 ppm could require cumulative emissions over the 21st century to be less than ~1800 [1370 to 2200] GtCO2, which is about 27% less than the ~2460 [2310 to 2600] GtCO2 determined without consideration of carbon cycle feedbacks.” A significant body of research subsequent to AR4, especially with respect to the resilience of natural carbon sinks, has suggested that carbon cycle feedbacks may be more potent than previously thought.
Yet the most significant issue surrounding the 2°C warming limit may be the perception among some negotiators that it represents the upper bounds of warming at which disruption of ecosystems and societies would be minimal. Such perception is at odds with most projections; for example, the International Scientific Congress on Climate Change suggested in March that while some societies might be able to cope with a “two-degree world” through aggressive adaptive strategies, impacts could be “significant” even below 2°C.
Again, there’s much more to this than I can quote here, so please click on through.
There is a spreading view that we should be aiming for mo more than 1.5°C of increase, but, to no one’s surprise: U.S. Rejects Tougher Goal.
Perhaps it’s a good idea to step back and try to reconstruct the chain of logic here and try to see how the pieces of the puzzle interlock and depend on each other.
First and most obviously, we want climate change to cause less than a specific threshold of human impact. However you measure such a thing–and I’m not about to step into that quagmire–call the quantity I.
Next, we need to know what amount of warming, call it W, translates to I units of impact, via function f. Therefore, impact = f(W).
Finally, we need to know how much greenhouse gas we can emit to cause no more than the maximal amount of warming. Call this E, and function g gives us warming = g(E). Note that this is the “area under the curve”/total carbon budget view of the problem, which I’ve argued is the most logical and accessible way of talking about the problem.
Our situation boils down to figuring out what E is, such that:
f(g(E)) <>
This looks simple enough at such a high level of abstraction. But peering into the details reveals considerable nastiness.
Function g, which maps greenhouse gases to warming, is complicated by feedbacks and long (by human time frames) time lags. As the climate warms, ice sheets and polar ice melt, lowering the albedo of some areas and making the planet absorb more heat from the sun. A warming ocean also absorbs less CO2, somewhat limiting its ability to temper the effects of our emissions. This is also where the Archer bonus comes into play, the 40% of warming from our emissions up to 2100 that we’ve arbitrarily decided don’t matter.[1]
Speaking of feedbacks, some news was released Tuesday: (NASA Outlines Recent Breakthroughs in Greenhouse Gas Research) (emphasis added):
Researchers studying carbon dioxide, a leading greenhouse gas and a key driver of global climate change, now have a new tool at their disposal: daily global measurements of carbon dioxide in a key part of our atmosphere. The data are courtesy of the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua spacecraft.
…
In another major finding, scientists using AIRS data have removed most of the uncertainty about the role of water vapor in atmospheric models. The data are the strongest observational evidence to date for how water vapor responds to a warming climate.
“AIRS temperature and water vapor observations have corroborated climate model predictions that the warming of our climate produced as carbon dioxide levels rise will be greatly exacerbated — in fact, more than doubled — by water vapor,” said Andrew Dessler, a climate scientist at Texas A&M University, College Station, Texas.
…
“The implication of these studies is that, should greenhouse gas emissions continue on their current course of increase, we are virtually certain to see Earth’s climate warm by several degrees Celsius in the next century, unless some strong negative feedback mechanism emerges elsewhere in Earth’s climate system,” Dessler said.
In other words, water vapor is a much larger feedback and can therefore act as a much more powerful multiplier on the warming effect of CO2 than previously known. (Someone who knows climate science far better than I do has told me that scientists have suspected the water feedback was high for some time, and this merely confirms it, and is therefore nothing “new”.)
One way to visualize how a system responds to shocks (like the injection of hundreds of billions of tons of CO2 into Earth’s atmosphere), is to imagine a ball sitting on a table or at the bottom of a bowl or balanced on top of a dome. The ball’s initial position represents the state of our climate, more or less as human beings have enjoyed it for thousands of years as civilization has spread around the world and our numbers have grown to roughly 6.8 billion; the further the ball moves from the initial position, the less human friendly the climate. On the flat table, a little nudge sets the ball into motion until friction eventually stops it. In the bowl, a nudge pushes the ball, but it rolls back and forth and soon returns to essentially its starting position. On the dome, a slight push sends the ball rolling off and headed who knows where. The more we learn that the climate is “twitchy” in response to being nudged, the more convex (dome shaped) is the surface the ball rests on. NASA’s discovery (or confirmation) about water vapor means the surface under the ball in our greatly simplified model is a bit more convex than we thought.
In fact, the surface beneath the ball is probably more like a dome with a dimple forming a small bowl on top. A tiny nudge will make the ball oscillate a bit and stay in the dimple; too large a push and it goes over the edge of the dimple and down the side of the dome. In the real world, nature’s systems will compensate for minor changes, especially given enough time. But when presented by a sudden, sharp shock, as in our record of greenhouse gas emissions over the last couple of centuries, and all bets could be off.[2]
Never forget the monster under our bed, the potential for massive releases of methane and CO2 from permafrost and hydrate deposits, which could put us on a much taller, narrower dome than we thought. How much can we disrupt the environment before we send the ball over the edge, out of our cozy equilibrium state, and into the realm where those feedbacks overwhelm everything else is still unknown.
Clearly, the critical factor is just how much feedbacks add to the warming caused directly by greenhouse gas emissions. If it turns out that feedbacks have a huge contribution once we get over a certain level of warming (i.e. we hit one or more tipping points, which seems inescapable considering the permafrost melting and hydrate releases we’re already observing) then it is absolutely imperative that we understand where those tipping points lie; anything less would constitute playing Russian roulette, but without spinning the chamber in between trigger pulls. We wouldn’t know exactly when we’d cause a tragedy, but we’d be certain it would happen eventually.
Function f, mapping a level of warming to human impact, is also problematic to nail down, even assuming we’ve agreed on a way to measure something that complex.
Sadly, there’s new, and unpleasant, news here, as well.
Study: Earth’s polar ice sheets vulnerable to even moderate global warming:
A new analysis of the geological record of the Earth’s sea level, carried out by scientists at Princeton and Harvard universities and published in the Dec. 16 issue of Nature, employs a novel statistical approach that reveals the planet’s polar ice sheets are vulnerable to large-scale melting even under moderate global warming scenarios. Such melting would lead to a large and relatively rapid rise in global sea level.
According to the analysis, an additional 2 degrees of global warming could commit the planet to 6 to 9 meters (20 to 30 feet) of long-term sea level rise. This rise would inundate low-lying coastal areas where hundreds of millions of people now reside. It would permanently submerge New Orleans and other parts of southern Louisiana, much of southern Florida and other parts of the U.S. East Coast, much of Bangladesh, and most of the Netherlands, unless unprecedented and expensive coastal protection were undertaken. And while the researchers’ findings indicate that such a rise would likely take centuries to complete, if emissions of greenhouse gases are not abated, the planet could be committed during this century to a level of warming sufficient to trigger this outcome.
In other words, for a given level of warming it seems that we’ll get quite a bit more human impact than previously thought. Notice that some effects of warming, like melting glaciers and polar ice, are both a feedback and a direct human impact. Others, like methane and CO2 releases from Arctic regions, are feedbacks that only matter because of the effects of the additional warming they cause.
And the important detail of ice dynamics–exactly how those immense sheets of ice respond to a warming world–is still being unraveled, as described in Greenland glaciers: Water flowing beneath ice plays more complex role and Predicting future sea level rise.
Where does that leave us? I think it’s clear that the 2°C guardrail was based on nearly 40-year-old assumptions about how the world works, essentially the exact nature of our functions f and g, that are increasingly being shown to be either incorrect or highly suspect. We’re finding out that the climate is “twitchier”, meaning it likely has more of a tendency to lurch from one state to another and with less provocation than we thought. That doesn’t mean the “right answer” is 1.5°C; it could be between that level and 2°C, it could be even lower, and therefore closer to our current level of roughly 0.7°C increase.
We need a strong push in all parts of climate science so that we can understand and model these key interactions within our environment. For now, it’s inescapably clear, and becoming more so all the time, that we’re taking immense risks by not dramatically reducing our CO2 and methane emissions.
[1] Are so many people comfortable with this cutoff at the year 2100 simply because no one expects even their newborn children to live to that date, it’s a nice round number, and we really need that 40% bonus? I’d hate to think we’re that shallow and shortsighted, but I can’t explain it otherwise.
[2] To come even somewhat close to mirroring reality, the ball imagery would need a much more complex surface. The dome would likely be asymmetrical, and it would have terraces in various places that would catch the ball on its way down, representing new equilibrium states after an initial shock to the system. Even the dimple on top of the dome would likely be asymmetrical, reflecting that the initial state of the environment is more prone to some kinds of shocks (coming from different directions) than others.
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