Is the current increase in global surface temperatures human-induced, or is it part of natural climate cycles? Phil Edmunds (Letters, April 27) asks why glaciers at Yoho National Park in Canada receded during the late 1880s, before modern human activity was significantly influencing the environment.
Changes to the climate happen for a variety of natural reasons and on a variety of time scales. Between about 1400 and 1850, the North Atlantic region experienced what has sometimes been called the “Little Ice Age.” This cooling, caused by low solar output and more frequent volcanic eruptions, helped glaciers advance in parts of the Northern Hemisphere. When the cooling trend ended in those areas — due in part to natural factors, but increasingly, as a result of the growing impact of industrial CO2 emissions — glaciers began to recede. This is most likely why the Yoho glaciers were retreating in the 1880s.
The fact that natural changes have occurred in the past does not preclude humans from having an additional — and potentially much larger — impact on the climate today. The Little Ice Age’s impact was about 2 to 4 degrees in the affected regions, and only a few tenths of a degree globally. Fossil fuel-produced CO2 emissions have the potential to raise global temperatures by many degrees, depending on how much we consume. That’s why we need to quit quibbling about the science and start thinking about what we might do to address the problem.
Jim Kasting, Michael Mann and Lee Kump
The writers are Penn State professors with the Earth and Environmental Systems Institute.