The Projection Project
Short version: With high confidence, warming from rising carbon dioxide will bring more very hot days and fewer very cold ones, more sea-level rise, stress for endangered species, plant fertilization but heat stress, more-intense peak rains but drying in many times and places, and many other impacts. Small changes will bring winners and losers, but losers will grow to far outnumber winners if we continue on our current path and cause very large changes.
Friendlier but longer version: For the next decade or two, the biggest uncertainties about future climate are linked to things we cannot know—will there be a big volcanic eruption in the next decade, or an extra El Nino or La Nina? The expected warming over a decade or two for any of the choices we are likely to make is more-or-less the same size as the cooling effects of a big volcano or La Nina. For a small number of decades after that, the biggest uncertainties are probably linked to things we don’t fully understand about the climate. Recall that the equilibrium warming from doubled CO2 is estimated to be between 1.5 and 4.5°C. The big difference between the high and low estimates might be reduced by better climate science, although the interactions among feedbacks mean that greatly reducing the uncertainty is quite difficult. But, by late in the century, the uncertainties related to volcanoes or climate sensitivity are smaller than the uncertainties related to what we humans choose to do. And remember at least the younger students in this class are likely to live longer than that!
Because our choices are so important, climate scientists normally don’t discuss predictions, choosing instead to provide projections: “If people decide to do xxxx, then the climate will do yyyy, with an uncertainty of zzzz.” By replacing the “xxxx” with different things we might do, the science shows policymakers and other people the changes yyyy±zzzz that their decisions would cause.
Video: Past and Future CO2 Atmospheric Concentrations (1:58)
PRESENTER: This is another figure from the IPCC from 2007, their fourth assessment report. The year 1000 is over here. So this is year, and it comes up to the year 2000 and then into the future going that way.
This is how much CO2 was in the air. And so what we're looking at here is a long period of stability. These are ice core data from breaking bubbles in various cores in Antarctica with different levels of impurities, different snowfall, different temperature but the same record.
As we come in here, what we see is that the ice cores and the instrumental record, what's measured in the air today, actually agree just beautifully and that we really, really have raised CO2. And these are various possible futures running off here to the right. And depending on sort of how the economy grows and so on, none of these include a strong effort to reduce CO2. So far we've been tracking very near the highest of these or above it a little bit, but we haven't gone very far and so it's a little hard to tell which way we're going.
The things to notice are that the rise so far from human CO2 is unequivocal. It's beautifully clear scientifically, but it's not very big compared to what's coming in all the future's envisioned. We see a much larger change in the future than in the past.
And all of these curves are still headed up as they get to the point where students today are getting old but are still not passed away. And our children and our grandchildren very clearly will live off of this. So if we don't do something about our CO2, the changes coming are very, very much bigger than the changes that have happened so far.
The graph just above shows the history of atmospheric CO2 over the last millennium as measured in bubbles from ice cores, including the very close agreement between ice-core and atmospheric data during the decades of overlap, and then shows various possible futures. These future “scenarios” were prepared to bracket likely paths we may follow, and provide enough curves so that one of them may prove to be fairly close. So far, we’re running near the highest of the projections, but close to the others because the different scenarios don’t diverge a lot until further in the future. None of these paths assumes that we take major efforts to reduce greenhouse-gas emissions, which could lower any of them.
Notice that in all of these scenarios the projected changes are much larger than those to date, with CO2 still rising beyond 2100. (The world does NOT end in 2100!) With notable uncertainties, fossil fuels may become rare by the time CO2 reaches the top of the chart around 1000 ppm, or may be common enough to drive CO2 more than twice that high, giving us two or three doublings from the relatively stable level of approximately 280 ppm before the industrial revolution.
We could estimate future temperature by taking the climate sensitivity of around 3°C for doubled CO2 (or between 1.5 and 4.5°C), and the two or three doublings, calculating a warming, and reducing that a bit because the warming lags the CO2 a little and the CO2 will start down before peak warming is reached. We get much more information by taking our best models, run by different groups in different ways, forcing them with the scenarios, and studying the results.