Weart replies: Earth’s climate system involves many basic phenomena—science teachers should note how that could be used to spark interest! George Smith’s letter shows some ways a temperature rise can cause CO2 emissions. Such feedbacks are worrisome, because they could accelerate warming once it is initiated.
What initiated the current warming? It took many decades for scientists to agree on the most likely answer. 1 The crucial observation was the recent atmospheric CO2 increase, whose rate and magnitude are vastly beyond anything in the ice core record. The steep climb neatly matches calculations of the rise expected from the known consumption of fossil fuels. The calculations include estimates of gas exchanges with the oceans, tundra, forests, and so forth: estimates checked through many measurements—for example, of carbon isotopes. The oceans are found to be a net absorber, transporting carbon into their depths. Net biosphere output, although harder to estimate, is certainly dominated at present by emissions due to human activities. 2
Sea ice will melt provided the greenhouse effect adds enough energy to the planet to warm the sea-water even while the ice melts. A temperature increase has been observed, and will bring sea-level rise through thermal expansion.
