Are the climate changes we are seeing and measuring today due to natural cycles, or is this human-induced climate change? Lutricia Smeal (Letters, March 7 CDT) argues that, because the Earth has experienced natural climate changes over long time scales, humans cannot be the major cause of climate change today. These climate shifts certainly happened, but her analysis overlooks the mechanisms that caused them.
The warm climate of the dinosaurs is thought to have been caused by CO2 levels at least four to six times higher than today, a result of large amounts of the gas being released into the atmosphere as plate tectonics shifted the continents. CO2, of course, is increasing rapidly today from fossil fuel burning, and so the same type of warm climate could reoccur, albeit for different reasons.
Snowball Earth episodes billions of years ago happened because the young sun wasn’t as bright as today and because there wasn’t enough CO2 (or other greenhouse gases) to compensate.
Finally, the most recent Ice Age that brought glaciers to North America was driven by changes in Earth’s orbit, amplified by various feedback mechanisms, including 30 percent lower CO2 compared to the preindustrial Earth. Earth’s orbit is still changing, but far too slowly to offset the rate of increase in CO2. So, yes, climate does vary naturally. Yet our actions can still cause a potentially dangerous warming on top of this natural variability. Fortunately, we have the ability to make creative and positive decisions to change that future.
Jim Kasting, Jenni Evans, Klaus Keller and Lee Kump