The Mauna Loa CO2 Record: From the Era of Discovery to the Era of Consequences 2008 marks the 50th anniversary of the Mauna Loa and South Pole CO2 records, which are the longest continuous time series of atmospheric CO2 levels. These records have played a critical role in advancing research on global warming by establishing the reality of increasing CO2 and providing a quantitative basis to assess the impact of human activities on atmospheric CO2. From 1958 to 2008, the CO2 levels at Mauna Loa increased from 315 to 385 part-per-million. The records establish that an amount of CO2 equivalent to 56% of the global emissions of fossil-fuel burning over this period has been retained in the air. The remaining 44% has therefore been absorbed by the oceans and land plants. Our ability to predict the impact of future emissions on the CO2 loading of the atmosphere and hence future climate hinges critically not only on future CO2 emissions, but also on the behavior of these land and ocean sinks. Over the past decade, our understanding of these sinks has improved, based in part on observations of trends in atmospheric O2 concentration. Our knowledge of these sinks establishes securely that large reductions in fossil-fuel CO2 emissions will be required over the next few decades to stabilize CO2 below “dangerous” levels. Recent work has also raised concerns that the sinks may be weakening due to effects of global warming on the stores of carbon in land ecosystems or in the oceans. This subject remains clouded in uncertainty, however. Even larger and more immediate emissions reductions may be necessary if such “positive feedbacks” turn out to be important. Carbon Dioxide in Historical Context: Implications for Policy The uptake of fossil fuel into the biosphere is limited, both in how fast the carbon will be taken up, and in the total amount of CO2 that will be absorbed, by the ways in which the carbon cycle on Earth works. The carbon cycle today is taking up fossil fuel CO2, slowing considerably the rate of CO2 rise and warming. But CO2 concentration measurements from ice cores from the past 800,000 years suggest that ultimately the carbon cycle may act as an amplifier of climate change, releasing carbon during times of warmer climate. The biosphere on land is currently in net balance, with natural uptake in some areas compensating for deforestation in other areas. The land biosphere could act as either a source or a sink in the coming century, but ultimately would be swamped by the amount of fossil fuel carbon available. About three quarters of of the carbon we release will dissolve in the oceans on a time scale of a few centuries. Uptake into the oceans will slow as the rising CO2 concentration exhausts the buffer chemistry of seawater, its ability to dissolve more CO2. CO2 is also less soluble in warmer water than cold, so that CO2 uptake will decline further with climate warming. There is recent evidence that CO2 uptake in the Southern Ocean, the main invasion route into the deep sea, has been slowing even more quickly than expected based on those two reasons alone, suggesting that CO2 uptake into the ocean is also slowing because of changes in ocean circulation. The ocean might take up CO2 more slowly if its overturning circulation stagnates in a warmer world. Carbon cycle models agree that even after the ocean and land have taken their fill of fossil fuel CO2, between 15 and 30%, will remain in the atmosphere for thousands of years. Many of the most profound changes in Earth's climate will take place on these long time scales, such as the melting of ice sheets, permafrost soils, and methane hydrates in the ocean. Sea level in the past has changed by 5 to 20 meters for each degree C change in Earth's temperature. These results imply that the long-term change in sea level from fossil fuels could be