If carbon dioxide levels in Earth’s atmosphere are allowed to rise beyond what they are today, certain effects to the Earth’s climate will be “locked in” and irreversible for at least a thousand years, a new study finds.

"Our study convinced us that current choices regarding carbon dioxide emissions will have legacies that will irreversibly change the planet," said study leader Susan Solomon, of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo.

Climate scientists have known for some time that a portion of the carbon dioxide emitted now will stay in the atmosphere for thousands of years. This is because the ocean is slow to absorb both the carbon dioxide and heat from the atmosphere; and as the ocean warms, it can absorb less carbon dioxide.

The two uptake processes "work against each other to keep temperatures almost constant for more than a thousand years, and that makes carbon dioxide unique among the major climate gases," Solomon said.

The new study, detailed in the Jan. 26 issue of the journal Proceedings of the National Academy of Sciences, showed how this lingering carbon dioxide could affect the Earth's climate systems.

Solomon and her colleagues used measurements and models to show how changes in surface temperature, rainfall, and sea level are largely irreversible for more than 1,000 years after carbon dioxide levels reach a range of peak concentrations and emissions are subsequently completely stopped.

Carbon dioxide levels in the atmosphere today are about 385 parts per million (ppm) (in other words, for every million molecules of gas in the atmosphere, 385 are carbon dioxide molecules). Before the industrial revolution, carbon dioxide levels were around 280 ppm.

Droughts and sea level rise

If carbon dioxide 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 Dust Bowl in the American west and in zones including southern Europe, northern Africa, southwestern North America, southern Africa and western Australia, the study found.

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.

The scientists emphasize that increases in carbon dioxide that occur in this century "lock in" sea level rise that would slowly follow in the next 1,000 years.

Considering just the expansion of warming ocean waters — without melting glaciers and polar ice sheets — the authors find that the irreversible global average sea level rise by the year 3000 would be at least 1.3–3.2 feet (0.4–1.0 meter) if carbon dioxide peaks at 600 parts per million, and double that amount if it peaks at 1,000 parts per million.

And this is just the minimum sea level rise at those carbon dioxide levels — melting ice could add more to the total but weren't included in the study because they are less well-understood.

"We presented the minimum sea level rise that we can expect from well-understood physics, and we were surprised that it was so large," Solomon said.

Rising sea levels would cause "irreversible commitments to future changes in the geography of the Earth, since many coastal and island features would ultimately become submerged," the authors wrote.