Earth's Fidgeting Climate
Is human activity warming the Earth or do recent signs of climate change signal natural variations? In this feature article, scientists discuss 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.
In spite of what may seem persuasive evidence, many scientists are nonetheless skeptical.
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 their 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.
The canary in the coal mine?
The recent discovery that Greenland's ice sheet is thinning is a good example of our climate's sometimes vexing ambiguity.
Many newspaper headlines cried the discovery as a sign of global warming -- which most readers presumably took to mean "anthropogenic," or human-caused, global warming.
But is that the right conclusion?
"What you can say is, yes, carbon dioxide (in the atmosphere) is at levels higher than ever before, and carbon dioxide is a greenhouse gas, so it's reasonable to say that there's warming associated with the increase of carbon dioxide," said Dr. Waleed Abdalati, co-author of the paper that announced the Greenland discovery.
"But you can't make the leap yet that all the cars in the world have led to what we're observing in the thinning of the Greenland ice sheet," Abdalati said.
If there's one lesson to be learned from science, it's that things are usually much more complex than they at first appear. The warming trend of the last century may seem to be the obvious explanation for the thinning seen on Greenland, but scientists are considering other possibilities.
Last century's warming trend is not the only possible explanation for the thinning that Krabill's team saw on Greenland.
In fact, ice cores taken as part of another NASA-funded study suggest that natural variation in snowfall may be partly to blame, Mosley-Thompson said.
Other natural processes could account for the thinning as well. Ocean currents might have caused part of the change. Or the flux of warm water into the North Atlantic caused by the 1990-1996 positive phase of the slow-moving North Atlantic Oscillation could have had an influence. The ice sheet could also be thinning in response to the long-term warming of the planet since the transition from the last glacial period about 10,000 years ago. Krabill, Dr. Ron Kwok of NASA's Jet Propulsion Laboratory, and Abdalati mentioned these scenerios during interviews with Science@NASA.
This natural variability often shows an astounding degree of complexity, much of which remains poorly understood.
"We've only begun making (large scale) measurements in the last 100 to 150 years," Abdalati said. "And climatic processes happen on very different time scales. There are some, like ice ages, that are in the tens of thousands or hundreds of thousands of years long. An then there are atmospheric processes like weather, which happen on the scales of hours and days."
Other climate cycles fall in between, such as the North Atlantic Oscillation mentioned above, which is thought to complete one cycle roughly every 20 to 30 years.
"And so you have all these processes mixed together that have been going on for thousands of years, and you're in the difficult position of trying to separate something very recent from the natural cycle without fully understanding what that natural cycle is," Abdalati said.
Knowing where a relatively short interval of observation fits into the long-term pattern is a difficult challenge for scientists. A steady increase that appears to be a trend may be a trend, but it may also be a small part of a larger cycle.
Seen in this context, scientists don't give much weight to the five-year snapshot of the ice on Greenland.
The problem with the "P" word - Proof!
From a statistical point of view, no single scientific result based on real-life data ever deserves absolute confidence. There always lingers the possibility -- however small -- that the apparent results are due to chance patterns in the data, i.e., "noise."
In addition to the caveats inherent to statistics, conclusions from studies like Crowley's that are based on computer simulations of the world's climate are plagued by questions of how well computer models portray the real thing.
To prove causation, scientists must perform experiments under controlled conditions on the system being studied, manipulating the system to understand what causes what. Other scientists repeat the experiments to show that the explanation is reliable.
Since the Earth's climate is beyond the reach of such experimentation, scientists instead run computer simulations of global climate. These models are always much simpler than the Earth's climate itself. In fact, it's theoretically impossible to create a "perfect" model of climate that includes all the detail of the real system.
"The climate system is too complex," Mosley-Thompson said. "Even the most complex climate model doesn't get it right. And why is that? Because who writes the climate models? Humans. What is a climate model? It's a set of equations that describes what we think we know. If you're not cognizant of a particular phenomenon, then how can you incorporate it into a climate model?"
Much of the energy coming from the Sun is in the form of short-wavelength radiation, which passes through the atmosphere. The Earth's surface re-radiates that energy as long-wavelength radiation -- such as infrared -- which is trapped by greenhouse gases, primarily water vapor, carbon dioxide and methane.
Because such conclusions are based on a scientist's professional judgment, disagreement is inevitable.
"There are always people -- and reasonable people -- who fall on both sides of the argument," agreed Abdalati.
Earth's Fidgeting Climate - NASA Science News
Greenland's Receding Ice
- Information, images and animations about the discovery of thinning ice on Greenland by NASA scientists. Produced by NASA's Goddard Space Flight Center.
The North Atlantic Oscillation
-- An informational Web site from Columbia University.