Earth's Fidgeting Climate

Is human activity warming the Earth or do recent signs of climate change signal natural variations? In this article, Patrick L.Barry from NASA discusses the vexing ambiguities of our planet's complex and unwieldy climate.

Newspaper headlines trumpet record-breaking temperatures, dwindling sea ice, and retreating glaciers around the world. Concentrations of atmospheric carbon dioxide, one of the greenhouse gases responsible for scalding temperatures on Venus and at least 33 degrees C of normal warming here on Earth, are on the rise. Our planet seems destined for a hot future!

But is it really? Or are we simply experiencing a natural variation in Earth's climate cycles that will return to "normal" in time?

Correlations between rising CO₂ levels and global surface temperatures suggest that our planet is on a one-way warming trend triggered by human activity. Indeed, studies by paleoclimatologists reveal that natural variability caused by changes in the Sun and volcanic eruptions can largely explain deviations in global temperature from 1000 AD until 1850 AD, near the beginning of the Industrial Era. After that, the best models require a human-induced greenhouse effect.

They argue that natural variations in climate are considerable and not well understood. The Earth has gone through warming periods before without human influence, they note. And not all of the evidence supports global warming. Air temperatures in the lower atmosphere have not increased appreciably, according to satellite data, and the sea ice around Antarctica has actually been growing for the last 20 years. It may surprise many people that science -- the de facto source of dependable knowledge about the natural world -- cannot deliver an unqualified, unanimous answer about something as important as climate change.

Why is the question so thorny? The reason, say experts, is that Earth's climate is complex and chaotic. It's so unwieldy that researchers simply can't conduct experiments to check their ideas in the usual way of science. They often rely, instead, on computer models. But such models are only as good as there inputs and programming, and today's computer models are known to be imperfect.

Most scientists agree that no single piece of data will likely resolve the global warming debate. In the end, the best we can expect is a scientific consensus based on a preponderance of evidence.

Global Average Temperatures 1860-2000

The canary in the coalmine?

The recent discovery that Greenland's ice sheet is thinning is a good example of our climate's sometimes vexing ambiguity. About 85 percent of Greenland is covered by a massive ice sheet with an area of about 1,736,000 square kilometres and an average thickness of about 1,500 meters. The volume of ice in the Greenland sheet is estimated to be about 2,600,000 cubic kilometres -- enough ice to raise sea levels by 6.4 meters if it all were to melt.