Are people changing the climate? Should we worry about rising sea levels? Does global warming cause wildfires? We all have opinions on climate change, but understanding the science can inform our thinking. The first column in the series described some less debated aspects; this column discusses misconceptions and disagreements.
When a city has a record low or the global temperature declines one year, is this evidence against global warming? No, because variability is expected. Individual years can be cooler than the year before, even as the long-term trend is upward. There’s also variation across Earth’s surface. The “global temperature” is an average, and some parts are warmer than others. This means individual cities could have a low temperature even if the planet as a whole is warmer than before.
The planet is warmer; there’s no disagreement about that. The big debate is over cause: are people changing the climate, or is it natural? Humans’ greenhouse gas emissions are “extremely likely to have been the dominant cause of the observed warming since the mid-20th century,” according to the Intergovernmental Panel on Climate Change’s (IPCC) fifth assessment report. This means warming before 1950 could be due to natural cycles, but the scientific consensus says human activity has raised the temperature about one degree Fahrenheit since 1950.
Should we worry about a planet that’s one degree too warm? Possibly, but the consequences are often exaggerated. The Union of Concerned Scientists claims “global warming is already having significant and harmful effects,” citing sea level rise, wildfires, heat waves, droughts and extreme storms. Those sound scary, but many of them are small or only weakly tied to global warming.
Sea level rise is small and partly natural. The geological record shows oceans have been rising since the last glacial stage, so human activities aren’t the only factor. Since 1950, human-caused global warming has melted land ice in Greenland and Antarctica, according to the National Snow and Ice Data Center. Sea level has risen five inches since 1950, and the National Oceanic and Atmospheric Administration says it’s rising about 0.12 inches per year. While five inches isn’t huge, the continuing trend could be serious.
There could be a link between wildfires and global warming, but it’s hard to tell. According to the National Interagency Fire Center, the number of wildfires each year in the US is fairly constant. The area burned has approximately doubled since 1980, but many factors are involved. For example, fires have historically been suppressed in the US. This means vegetation builds up and fires tend to be large when they do burn.
It’s hard to show that global warming causes heat waves, droughts and floods. Twenty-six climate scientists writing in the June 2013 Journal of the American Meteorological Society concluded heat waves are a little more frequent, but don’t follow the decade-scale trends of warming. Droughts and floods are too variable for definite conclusions.
Global warming is not proven to cause extreme weather. Berkeley scientist Richard Muller explained in his recent book, “Physics and Technology for Future Presidents”, that technology makes it easier to find storms. When the data are corrected for this observational bias, hurricanes and tornadoes aren’t actually increasing.
If the temperature hasn’t risen much and the consequences are small, is everything fine? Not necessarily. Climate change isn’t dramatic today, but it’s disturbing because of potential future effects. Next week’s column will close the series by discussing these risks.
High waves batter a beach in South Kingstown, RI, in this US Geological Survey photo. Sea level rise could be one of the most dangerous effects of climate change.
