Some experts argue that a warmer planet will make the oceans less capable of absorbing carbon, and thus accelerate climate change.
There are several proposals for developing artificial sinks to sequester carbon and lock it up until the problem goes away. Enrich the oceans with iron, for example, and they should be able to take up more carbon. But Houghton isn't convinced.
"The technical and economic aspects of an operational sequestration program require considerable research," he writes.
In other words, don't bank on it.
While many of the most important questions remain unanswered, progress has been made in some areas. In the early years, scientists didn't even include clouds in their computer models, because nobody knew how to do that. But perhaps the greatest advance has been a clear and candid recognition throughout the scientific community, and in the populace as well, that this is a real problem, anthropogenic in nature, and it isn't going to go away.
And in fact, climate change could accelerate dramatically as it begins to feed upon itself, as some experts have predicted.
"The natural processes on land and in the oceans that have removed carbon dioxide from the atmosphere for the last century may be starting to weaken," Houghton says.
If so, we've turned a major corner, creating a planet that can no longer heal itself.