CLIMATE CHANGE - DIGGING A LITTLE DEEPER
Variations in the behavior of the weather over long time periods, such as from one century to another, is referred to as climate change. Climate itself adjusts from times of ‘ice ages,’ when huge ice sheets covered large areas that are currently ice-free, to periods similar to today when ice sheets are largely confined to Antarctica, Greenland, and the floating Arctic sea ice. Paleo-climate records indicate that much of the climatic changes over the last two million years have occurred in a rather cyclical manner; with glacial periods lasting roughly 100,000 years with warmer interglacial periods of 10,000 years occurring in between.
Prior to the 1990s, scientists largely believed that the shifts in climate between ice ages and warmer periods occurred over centuries and millennia due to the large amount of time necessary to build up or melt an ice sheet over a kilometer in thickness. Geologic evidence from the last few decades, however, shows that there have also been rather abrupt periods of climate change spanning anywhere from years to decades. Abrupt climate changes can occur when certain variables change gradually pushing the Earth’s system across some limit of instability.
Climate variation occurs as a response to climate forcings which can cause either a warming or a cooling of the atmosphere. Over most of the Earth’s history the forcings have been entirely natural, caused by continental drift, variability in solar radiation, changes in the Earth’s orbit, and volcanic emissions. However, since the Industrial Revolution, human activity has had an impact on the global climate system, increasing the amount of greenhouse gases in the atmosphere, trapping heat and contributing to an overall global warming.
The Intergovernmental Panel on Climate Change (IPCC) reported that the Earth’s average surface temperature during the 20th century increased approximately 0.6°C. While this may seem like a small change, global temperatures are generally quite stable. The difference between global temperature today and the average global temperature of the last ice age is only about 5°C. However, over the last century we have also witnessed a nearly 10% decrease in snow cover and a 10-15% decrease in spring and summer sea-ice in the northern hemisphere. Other observed changes that have been linked to climate change include longer growing seasons, increases in rainfall and rainfall intensity in the northern hemisphere, and shifts in when ice freezes and breaks up on rivers and lakes.