The first difference is thatcarbon dioxide is not like other pollutants.
To make this clear, let’s use the old bathtub analogy. The faucet is the source of the pollutant. The tub is the environment. And the drain represents the means by which the pollutant exits the environment. The key fact to remember: The damage to public health is determined by the total amount of pollutant in the tub.
Take a familiar air pollutant like particulate matter. We are spewing it into the air from tailpipes and smokestacks (the faucet). It leaves the air through simple gravity (the drain). Most of it falls to earth in days or weeks.
So when it comes to the particulate-matter bathtub, the drain is very large. We can reduce the total level of particulate matter in the tub any time we want; all we have to do is turn the faucet down, or off, and the tub will drain rapidly.
Carbon dioxide is not like that. Once it’s in the tub, it stays there for up to 100 years before it drains out. And the drain in the bathtub (so-called “sinks” that absorb carbon out of the air, like oceans and forests) is comparatively small relative to the enormous amounts coming out of the faucet. And by the way, we’re actively making the drain smaller by cutting down forests and carbon-loading the oceans.
This makes for a very different situation. Even if we cut our emissions by a third tomorrow, we would still be increasing the total amount in the bathtub:
The typical climate-policy targets that get thrown around — reducing emission rates by 80 percent by 2050, for example — are relatively meaningless. They focus on the rate of flow from the faucet. But that’s not what matters. What matters is the amount in the tub. If the tub fills up enough, global average temperature will rise more than 2 degrees C and we’ll be in trouble. Avoiding that — staying within our “carbon budget” — is the name of the game.
The public-policy implications are straightforward: Because CO2 is slow to drain, and the damages are cumulative, we need to reduce the amount of CO2 we’re spewing out of the faucet now, as much as possible, as quickly as possible. Yes, we’ll need new technologies and techniques to drive emissions down near to zero, and we should R&D the hell out of them. But we absolutely cannot afford to wait. There is no benign neglect possible here. Neglect is malign.
The second difference is that climate change is irreversible.
As Joe Romm notes in a recent post, New York Times columnist Joe Nocera slipped up in his latest column and referred to technology that would “help reverse climate change.” I don’t know whether that reflects Nocera’s ignorance or just a slip of the pen, but I do think it captures the way many people subconsciously think about climate change. If we heat the planet up too much, we’ll just fix it! We’ll turn the temperature back down. We’ll get around to it once the market has delivered economically ideal solutions.
But as this 2009 paper in Nature (among many others) makes clear, it doesn’t work that way:
This paper shows that the climate change that takes place due to increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop. Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing, but is largely compensated by slower loss of heat to the ocean, so that atmospheric temperatures do not drop significantly for at least 1,000 years. [my emphasis]
This is not the time cycle of particulate pollution — days or weeks — it is the time cycle of the Earth’s basic biophysical systems, which move much more slowly. A thousand years is not “forever,” but in terms of human agency it might as well be.
The damage we’re doing now is something the next 40 to 50 generations will have to cope with, even if we stop emitting CO2 tomorrow. And the CO2 we’ve already released has locked in another 50 or 100 years of damage (because of the slow draining). There is no “reversing” climate change. There is only reducing the amount we change the climate.
Both these facts about climate change set it apart from other environmental problems. They also, for what it’s worth, set it apart from social problems like poverty, crime, or poor health care. All of those problems are serious; they all have an impact on public health. But they can all be measurably affected by public policy within our lifetimes. They are bad but they are not cumulative. They are not becoming less solvable over time.
Climate change, on the other hand, is forever.