Rep. Tom McClintock (R-N.Y.) said, "I was struck by his noting that the glaciers in Yosemite were disappearing. It occurred to me that had he given that speech on that very spot 12,000 years before, he would have been covered by nearly 3,000 feet of ice. Doesn't that pre-date the invention of the SUV? I think we can agree that global warming has been going on for a long time. It has been going on and off since the last ice age."
Michael Mann, a climate scientist at Pennsylvania State University, said: "Those statements betray a profound ignorance of what the science has to say. They confuse anecdotal claims from individual regions with quantitative evidence of past climate changes." Daniel Jacob, a professor of atmospheric chemistry and environmental engineering at Harvard University, added: "It's happening so fast, that's what is unique. Those of us who have lived long enough have personal experience of warming having happened over our lifetimes (less snow, later pickup of leaves, warmer temperatures in our favorite swimming hole, etc.). And the data show that the trend is accelerating. We know from ice cores that climate has never changed that fast in the past million years."
Rep. Doug LaMalfa (R-Calif.) asked, "How do you explain the fluctuations in CO2 levels pre-Industrial Revolution?" Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, says "The carbon cycle is fundamentally tied to both the climate and to life itself and one should expect that factors that affect either will also affect the carbon cycle. Over very long time (multi-million year) timescales, carbon dioxide levels are set from a balance between volcanic gases and weathering of exposed rocks."
Jacob added: "CO2 responds to changes in vegetation and to changes in the ocean. So as the planet changes the CO2 concentration also changes. For example, CO2 is lower during glacial climates, and it was higher than today at the time of the dinosaurs. We know from ice cores that the CO2 concentration today is higher than it has been for the past 800,000 years. Also that the rate of increase in CO2 over the past century is unprecedented."
LaMalfa also asked, "What percentage of the CO2 production in the world today is caused by what people do versus what the planet itself does?" Jacob said, "Only a small percentage, but that is what causes the trend. The natural sources and sinks of CO2 are in equilibrium but the additional CO2 emitted by people doesn't all get taken up — half stays in the atmosphere and accumulates."
Schmidt said: "The natural cycle of CO2 was roughly in balance for the last 10,000 years or so — with inputs of CO2 into the atmosphere balancing removal of CO2 into the land biosphere and ocean. Estimates of this natural flux are around 90 gigatons of carbon in each direction per year. Current human-caused additions from fossil fuel burning and deforestation are around 10 gigatons of carbon per year only a small part of which is being balanced by increased removal. Thus each year the amount of CO2 in the atmosphere is increasing by about 2 to 4 parts per million (ppm) due to the current imbalance. The net effect of human activity has been to raise CO2 levels from 280 ppm to over 400 ppm today — an over 40 percent increase." (Read more)