How Much Warming Are We Asking For?

"A New Epoch"

A recent report from the U.S. National Research Council does not mince words.

"Emissions of carbon dioxide from the burning of fossil fuels have ushered in a new epoch where human activities will largely determine the evolution of Earth’s climate. Because carbon dioxide in the atmosphere is long lived, it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe. Therefore, emissions reductions choices made today matter in determining impacts experienced not just over the next few decades, but in the coming centuries and millennia." [From the executive summary]

Key points of the report:

• Because CO₂ persists so long in the atmosphere, our emissions today and over the rest of the century will have profound effects on Earth's systems for centuries or even millennia to come. 
• To achieve atmospheric CO₂ stabilization at any given level, emissions will have to be cut by more than 80% from current rates. The longer we wait to make those cuts the higher the level will be when it stabilizes.
• Global average temperatures can be correlated with atmospheric levels of carbon dioxide, so we can make predictions of future warming from projections of future emissions.
• A growing body of research allows specific likely changes in Earth's systems to be forecast based on projected average global temperatures.
• "The report concludes that certain levels of warming associated with carbon dioxide emissions could lock the Earth and many future generations of humans into very large impacts; similarly, some targets could avoid such changes."

Projected climate impacts

The report, "Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia", addresses this situation by providing estimates of the impacts from various levels of global warming and CO₂ emissions. The authors hope policymakers will take advantage of this information.

The report estimates changes in precipitation, streamflow, wildfires, crop yields, and sea level rise that can be expected with different degrees of warming. It also estimates the average temperature increases that would be likely if CO2 were stabilized in the atmosphere at various target levels. However, the report does not recommend any particular stabilization target, noting that choosing among different targets is a policy choice rather than strictly a scientific one because of questions of values regarding how much risk or damage to people or to nature might be considered too much. [From the press release]

(The full report is available here You have to enter an email address on a form to download the free PDF. A PDF of executive summary here.)

The report tries to convey a sense of urgency by reminding us that "climate change due to carbon dioxide will persist many centuries". This is because CO₂ remains so long in the atmosphere. Even if we were to reduce our emissions immediately the excess CO₂ we have already put into the atmosphere will continue to drive changes in the Earth's climate for decades or even centuries to come.

And the authors note that "Depending on emissions rates, carbon dioxide concentrations could double or nearly triple from today’s level by the end of the century, greatly amplifying future human impacts on climate." Translation: if you think the consequences we have already set in motion by past and current emissions are unattractive, just imagine the impact of annual emissions several fold higher continuing for many decades.

The Future

What will those quantities of carbon dioxide mean in terms of actual warming, and what will that warming do to the environment and society? First, the report estimates the warming we are buying with our current and future emissions. This table shows estimated eventual warming at various "stabilization" levels of CO₂. (Today's level is about 390 ppm, but of course we are not stabilizing at that level.)

Here are some of the corresponding impacts the report predicts:

• "For each degree (°C) of global warming:
  
  
  
  • 5-10% changes in precipitation in a number of regions
  • 3-10% increases in heavy rainfall
  • 5-15% yield reductions of a number of crops
  • 5-10% changes in streamflow in many river basins worldwide
  • About 15% and 25% decreases in the extent of annually averaged and September Arctic sea ice, respectively
• For warming of 2°C to 3°C, summers that are among the warmest recorded or the warmest experienced in people’s lifetimes, would become frequent.
• For warming levels of 1°C to 2°C, the area burned by wildfire in parts of western North America is expected to increase by 2 to 4 times for each degree (°C) of global warming."

We can fix it, right?

To make warming level off at any particular global temperature, the concentration of carbon dioxide would have to be stabilized at some level. But because human-caused emissions are rising sharply, and because past emissions will linger in the atmosphere and have continuing effects, annual emissions would have to be cut by at least 80% to get our production of greenhouse gases back in line with what the Earth's systems can deal with. Only by keeping emissions at the level the planet can absorb can we stabilize the atmospheric concentration of CO₂.

Note: this is only talking about stabilizing the atmospheric concentration of carbon dioxide at some higher level in the future. There is no way to turn back the emissions clock to get to lower levels of CO₂ that would commit us to less warming. (The report explicitly didn't consider "geoengineering" to get carbon out of the atmosphere.)

So far, some rich nations have collectively proposed to cut emissions by about 12.5% below 1990 levels by 2020 (see this previous post). Other countries, however, plan to keep increasing emissions. And the promised cuts are not firm. Some have not even been officially embodied in law or policy. There is still vague talk about cutting emissions 50% below 1990 levels by 2050. But even that would result in more CO₂ accumulating in the atmosphere every year. Our 1990 emissions, about 20 gigatonnes of carbon dioxide, were already far more than natural systems could absorb, and even half that level would exceed the capacity of natural sinks and result in continued accumulation.

So 50% cuts from 1990 global carbon dioxide emissions would not make atmospheric CO₂ concentrations level off. This report considers what warming would result when and if we eventually get CO₂ in the atmosphere to stabilize at some future higher level. If we don't achieve such stabilization, warming will continue beyond the temperatures discussed in this study.

We probably can't go back again to earlier historical CO₂ levels. The only question is what level we can get those CO₂ levels to stabilize at, if any. That will determine what degree of global warming we will eventually have.

Our legacy

If you think the heat waves and other very minor consequences we have seen with just one degree of warming are a cause for concern, you ain't seen nothing yet. We are already committing our great grandchildren to several degrees of additional warming--maybe three or four more degrees--a very different Earth. The question is how much we further we are willing to go beyond that.


[All quotations and images are from "Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia" by the Committee on Stabilization Targets for Atmospheric Greenhouse Gas Concentrations of the Board on Atmospheric Sciences and Climate Division on Earth and Life Studies, National Research Council of the National Academies. Published by The National Academies Press, prepublication copy, at http://www.nap.edu/catalog/12877.html. The report is copyright © National Academies of Sciences.]