Reducing emissions to zero might not be that big a problem if we had hundreds of years to do so. The problem is that we must reduce emissions to zero in the near future.

We cannot put a precise date on when emissions need to be zero. There is considerable uncertainty about the extent of temperature increases, harms from those increases, and the costs of reductions. No human has ever lived in a world where temperatures are on average, say, 3°C or 4°C warmer than they are today, not to speak of 5°C or 6°C, so we have little ability to predict what such a world would be like. We also do not know the costs of reducing emissions.

Current global agreements call for limiting temperature increases to 2°C. This may be unrealistic and perhaps a more realistic goal is 2.5°C or 3°C. Targets in this range, however, are not very far away. Consider Figure 6.1. Suppose our target is 2°C, we use the central estimate for the climate sensitivity, and we accept policies that give us only a 50% chance of meeting this target. This means that we can emit at most one trillion tons of carbon. We have already emitted about 580 billion tons of carbon and are emitting about 9 billion tons more each year (which means more than 30 billion tons of carbon dioxide). Even if the pace of emissions does not increase (and it has been increasing rapidly), we would hit the trillion-ton limit sometime between 2050 and 2060. If emissions rates increase, as they are likely to do without a change in policies, we will hit it even sooner with some estimates showing that we hit the trillion-ton limit before 2040.7

This timescale is based on accepting a 50/50 chance of exceeding a 2°C target and the central climate sensitivity. Let us see how much time we can buy by making the most optimistic assumptions. Assume that the climate is extremely insensitive to emissions, and say we only get 1°C of warming for a trillion tons of carbon. I am not aware of anyone who believes that the right target is greater than 4°C. It seems likely that the harm from temperature increases will start to go up rapidly at some point and almost certainly by the time we get to 4°C. At this level, we risk the collapse of major agricultural systems, leading to widespread disaster.8 Let us assume that we are, gulp, willing to accept a 4°C limit. The implied time limit with these assumptions will give us the longest possible period before emissions need to stop.

Even with these assumptions, if we continue on our current path, we hit the target before the end of the century. If we slow emissions, we buy more time because we use up the total more slowly. We can imagine a date sometime in the middle of the next century, and we only get this by assuming the lowest reasonable temperature increase for a given level of emissions, accepting the possibly terrible harms from a 4°C temperature increase, and by slowing emissions soon so that we get to our hard limit more slowly.

We cannot, however, count on these optimistic assumptions. If we end up with a high climate sensitivity (say, 3°C for one trillion tons), limit temperature increases to a 2°C, and do not slow emissions, we hit the limit in 2020, which is effectively tomorrow (or possibly yesterday, depending on when you are reading this).

The right target date is somewhere in this range. While we do not know the date, it is not easy to come up with a calculation that extends the time until we hit the target into the distant future. We need to be thinking about a 100-year horizon, and then only if we start slowing emissions soon.

Climate change is often portrayed as a very long-term problem, but our children and grandchildren will be alive near the end of the century and into the next, the time when we will face 2°C, 3°C, or even 4°C temperature increases. If we use middle-of-this-century targets, many of the current adults will be alive. While climate change will continue to affect people centuries in the future, it is a surprisingly near-term problem. Climate change is about people alive today, their children, and their grandchildren as well as the distant future.

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