First of two parts
Ninety-eight percent of climate scientists agree that our climate is changing.
Changes since 1950 include atmosphere and ocean warming, winter snow cover decreasing, Arctic sea ice decreasing, sea level rising, increased melting of alpine glaciers, increased atmospheric water vapor, and changes in the timing of seasonal events (earlier thaws, later frosts). Accompanying these changes are extreme rainfall events and drought.
Average global surface temperature has risen about 1.5 degrees Fahrenheit in the past century. The last decade was the warmest measured over the past 136 years. In the Northern Hemisphere, 1983-2012 appears to be the warmest 30-year period over the last 1,400 years. Sixteen of the last 18 months were the hottest on record; 2014, 2015, and 2016 were consecutively the hottest years on record.
Global ocean temperatures have steadily increased since the 1950s causing the ocean to expand, contributing to sea level rise. Rates of sea level rise have been greater during this time than the average rate during the previous 2,000 years.
Over the last two decades the Greenland and Antarctic ice sheets have been melting, as have most alpine glaciers. If the entire Greenland ice sheet melts, global sea level would rise about 23 feet.
Cause and effect
Human influence on climate change is clear. Emissions of greenhouse gases (GHGs) are the highest in history and are the reason for the recent rapid rate of climate change.
GHGs have increased to levels unseen in the last 800,000 years. During that time, GHGs and temperature were controlled by earth’s orbit around the sun. However, orbital changes are not behind today’s global warming; our current orbit indicates that the globe should be cooling not warming!
Atmospheric carbon dioxide (CO2) has increased approximately 40 percent since the 1700s, from 280 to 400 parts per million (ppm). The optimum CO2 level is believed to be 350 ppm or less.
Ocean acidification, caused by the absorption of atmospheric CO2, is the evil companion of climate change. The pH of the ocean is currently 8.1, which is 30 percent more acidic than 300 years ago. The ocean’s ecosystem would begin to crash if pH falls below 7.8. As acidification worsens, it would become more difficult for corals to construct reefs and mollusks to calcify their shells.
Reef building is particularly vulnerable to climate change, and current loss is a sign of that change. If current trends continue, coral reefs would cease to grow and start dissolving. Over the last 30 years Australia’s Great Barrier Reef has declined by 50 percent.
Scientists use models to test hypotheses, make predictions, and frame new questions. When Global Circulation Models (GCMs) do a good job of replicating past and current observations, scientists have confidence that these models can forecast the future.
GCMs are able to replicate the more rapid warming seen since 1950 as well as short-term cooling after recent volcanic eruptions (Mount Pinatubo 1991). GCMs also predict the extreme events we are currently experiencing such as major flooding and extended heat waves and drought. GCMs tell us about some of the drivers of large, sustained changes in global temperature observed over the last century.
Models using only natural forcings such as solar variation and volcanic activity and excluding GHG emissions show essentially no temperature increase over the last 100 years. But when the effects of human activity are included, the fit with observed data is very good. To successfully replicate climate behavior over the past century, models must include warming driven by CO2 emissions produced by human activity.
Coming next Wednesday: How climate change in the geologic past informs us about climate change today