Earlier this month in Paris, the Intergovernmental Panel on Climate Change released a 21-page Summary for Policy-makers based on its latest assessment report, the result of an exhaustive international scientific effort by hundreds of scientists lasting three years. In sobering terms, the summary concluded that recent warming of the climate system is “unequivocal” and that most of it is at least 90 percent certain to be due to human activities.
We climate scientists already knew this, but is the public convinced? The climate is changing, largely because people have added carbon dioxide, or CO2, to the atmosphere, mainly by burning coal, oil and natural gas. This extra CO2 adds to the Earth's natural greenhouse effect. As recently as the 19th century, the CO2 concentration was about 280 parts per million by volume, or ppmv, an amount virtually unchanged for thousands of years. Today it is about 380 ppmv and rising. The only way to halt the rise in concentration is to drastically reduce the rate at which we add CO2 to the atmosphere.
In any case, what should the ultimate goal be? Asking whether the right level at which to stabilize CO2 should be, say, 450 or 550 ppmv, may not be the best way to pose the question, at least scientifically. In fact, our scientific understanding of the climate system and its sensitivity to greenhouse gas concentrations is not yet good enough to say that 450 is “safe” but 550 is “dangerous.”
The risk goes up as the CO2 level goes up, and it's likely to be nonlinear. We may encounter various unpredicted tipping points along the way, any of which could be extremely nasty. These prevent us from predicting sea level rise with certainty, to pick one example, because processes governing the stability of the Greenland ice sheet are not yet well enough understood to permit firm quantitative projections. Thus, a preference for 450 or 550 ppmv or some other number is really a reflection of one's risk tolerance rather than a genuinely science-based decision.
Scientists can't make policy; they can only provide sound science as an input to wise policy. The tragedy is that policy-makers around the world have not yet responded with adequate urgency. There is very little sign of global determination to take this issue seriously enough to commit to stabilization of greenhouse gas concentrations at any level. What a state or city in the United States does is helpful, as are individual choices, but there is no meaningful national commitment, and that is true of most other countries as well.
Meanwhile, world population continues to grow, global energy demand continues to increase, and developing countries continue to exploit fossil fuels. Emissions of CO2 globally are rising, not falling. The imperative need is to generate public concern and political will that can start us on the path to rapid and massive reductions in CO2 emissions.
Stabilization of concentration at any reasonable level will take emissions cuts of something like 70 percent, which is 10 or more times the cuts envisioned under the Kyoto Protocol, a tentative first step diplomatically. We are effectively talking about weaning the entire world from fossil fuels.
There will be no single magic solution. We will need to pursue all the approaches vigorously. The most promising include energy conservation and efficiency, increased use of renewables, carbon sequestration and nuclear power. Research on advanced technologies not yet ready, such as fusion, must continue, as should efforts toward population stabilization.
I am optimistic about what technology can accomplish, once creative engineers and business people are encouraged by governments to work toward clearly stated goals of massive greenhouse gas-emission reductions. I am convinced that the economic case can be made convincingly, too, once people understand the cost of doing nothing or too little.
The question is not whether we, all 6-and-a-half-billion of us, have the technical capacity, but whether we have the collective determination to act. Settle that issue first, then decide on what CO2 concentration level is tolerable.
If we fail to act decisively and soon, then quibbling about targets of 450 versus 550 ppmv as a “safe” CO2 concentration will be meaningless. Procrastinating is simply asking nature to teach us what “dangerous” means, and that will certainly be an extremely unpleasant lesson.
Somerville is a climate scientist and distinguished professor at Scripps Institution of Oceanography at the University of California San Diego. He is a coordinating lead author for the most recent report of the Intergovernmental Panel on Climate Change. The views expressed here are his own.