“It has long been known that some of the carbon dioxide emitted by human activities stays in the atmosphere for thousands of years,” Solomon said. “But the new study advances the understanding of how this affects the climate system.”
The study examines the consequences of allowing CO2 to build up to several different peak levels beyond present-day concentrations of 385 parts per million and then completely halting the emissions after the peak. The authors found that the scientific evidence is strong enough to quantify some irreversible climate impacts, including rainfall changes in certain key regions, and global sea level rise.
If CO2 is allowed to peak at 450-600 parts per million, the results would include persistent decreases in dry-season rainfall that are comparable to the 1930s North American Dust Bowl in zones including southern Europe, northern Africa, southwestern North America, southern Africa and western Australia.
The study notes that decreases in rainfall that last not just for a few decades but over centuries are expected to have a range of impacts that differ by region. Such regional impacts include decreasing human water supplies, increased fire frequency, ecosystem change and expanded deserts. Dry-season wheat and maize agriculture in regions of rain-fed farming, such as Africa, would also be affected.
Climate impacts were less severe at lower peak levels. But at all levels added carbon dioxide and its climate effects linger because of the ocean.
The researchers also forecast a rise in sea levels of 2.3 to 3 feet, without factoring in melting glaciers or the polar ice caps because their models are too uncertain to quantify such changes (though I think we can safely say that more, rather than less, is likely.) As this NY Times article states, most researchers consider emissions in the range of 450 ppm to be "inevitable", and 600 ppm "difficult to avoid" without substantial changes in emissions:
“Policy makers need to understand,” he continued, “that in some ways once we are over the cliff, there’s nothing to stop the fall.”
Dr. Solomon said it would be wrong to view the report as evidence that it was already too late to do much good by reducing carbon emissions. “You have to think of this stuff as being more like nuclear waste than acid rain,” she said.
Acid rain began to abate when pollution contributing to it was limited. But just as nuclear waste remains radioactive for a long time, the effects of carbon dioxide persist.
“So if we slow it down,” she said, “we have more time to find solutions.”
Some critics argue that we can't forecast so far in the future because we can't anticipate the power of future technology to mitigate the effects of global warming. At the same time, neither should we assume that future technology will be able to deal with this problem, not when we can safely predict what the climate will do and can conceive only of outlandish schemes to deal with it.