In the mid 1950s, scientists began to seriously suspect that carbon dioxide released by burning fossil fuels was accumulating in the atmosphere and that this might have a noticeable effect on climate. But no one could say for sure. Some scientists thought the ocean would simply absorb the extra carbon dioxide, although if that was true, no one knew what the consequences of that might be either. Measuring airborne carbon dioxide was possible, but these measurements varied widely. Carbon dioxide emissions from cars and trucks skewed the readings from monitors placed near cities, and passing air masses could change them too. Analysis techniques were primitive, further obscuring the true values. Was it even possible to get a clean reading of overall atmospheric carbon dioxide? And what would the readings show?
The answer came from an unlikely place: high atop Mauna Loa, the largest volcano in Hawaii. Sitting in the midst of the Pacific Ocean, far from most industrial sources of CO2, this location looked perfect to scientist Charles David Keeling. In such a high, remote place, the atmosphere would be well-mixed. Keeling had also improved CO2 monitoring with better methods. Over the next few years, his data showed an inexorable zig-zagging increase of CO2. The oceans, it turned out, did rapidly absorb carbon dioxide, but most of the added gas quickly evaporated before the oceanic currents could pull it into the depths. But why was the line a zig-zag? The saw-toothed graph reflected the annual cycle of growth and decay in the Northern Hemisphere-trees use carbon dioxide from the atmosphere to grow leaves in the spring, and in the fall, the leaves decay and release carbon back into the air. Keeling's equipment at Mauna Loa has run nearly continuously since 1958. And every year, to a greater or lesser degree, the graph has ticked one notch higher.
Scientists can see the seas are rising, but in the past few decades, something troubled them about their sea level measurements: there was more of an increase than they could account for. Measurements of thermal expansion of seawater-the increasing space water takes up as it gets warmer-could only account for a fourth, and measurements of melting ice explained only a quarter or a third. What about the rest? One hypothesis is that measurements of ocean temperature are too sparse in dark waters below 10,000 feet and in the Southern Hemisphere, where parts of the ocean are harder to reach. Thus, unmeasured warmth in deep and southern waters may be throwing calculations off. A second possibility is that measurements of meltwater are insufficient, and that glaciers, ice sheets, and ice shelves are melting much faster than previously thought. A 2006 study by NASA scientist Eric Rignot seemed to bear that out by showing that the Greenland ice sheet was releasing almost twice as much water in 2005 as in 1996. Recently, scientists have found melt accelerating in other places too, including Antarctica and mountain glaciers. These rates are much higher than those predicted in the 2007 IPCC report. So as our data improves, the discrepancy is narrowing.
There are billions of time capsules locked in the ice of Greenland and Antarctica, but for years they were inaccessible to scientists, even though they could have held them in the palms of their hands. The ice trapped tiny air bubbles as it gradually accumulated, and scientists knew that inside lay ancient air that preserved the actual carbon dioxide content of the atmospheres of the past. Using drills, they bored holes deep into the ice to extract long, milky white cores. But frustratingly, 20 years of work failed to produce consistent, believable results. Finally, scientists realized the trick: thorough cleaning, crushing, and prompt measurement in a vacuum. Then in 1980, another revolution took place. A French and Russian team extracted a core over a mile (or two kilometers) long made up of 150,000 years of ice. Called the Vostok ice core in honor of the Russian Vostok Station in central Antarctica where it was taken from, it contained a complete glacial cycle of warmth and cold. Remarkably, the core showed that atmospheric carbon dioxide concentrations rose and fell with the temperature of the atmosphere. One ice drilling crew declared that the core "turned the tide in the greenhouse gas controversy." Though it didn't prove that changes in carbon dioxide concentration had caused the atmospheric temperature changes, it did at least show that carbon dioxide was an important piece of the climate feedback system that regulates our atmosphere.
One of the biggest surprises of the last few years is the discovery of an extensive network of rivers and lakes under the ice sheets of Antarctica and Greenland. In the 1970s, scientists flew radars over Antarctica to scan the bedrock below the ice and saw the undulations of the ground give way to extensive plane surfaces. They were stunned to realize there were lakes up to 20 miles long beneath the ice. One such, Lake Vostok, is the size of Lake Ontario. More than 160 subglacial lakes of varying size have been discovered so far. Where Antarctic glaciers moving seaward cross the lakes, the flow of the ice to the ocean seems to speed up. And these lakes can suddenly rise, drain, or flow into one another. Scientists aren't yet sure whether or how these lakes might affect the disintegration of ice sheets. If the ice sheets do fall apart, the two-foot sea-level rise the IPCC estimates may take place next century could become 20 feet or more. An increase of this magnitude would inundate the lower quarter of Louisiana and all of southern Florida and much of New York City, Boston, and Washington, DC would be flooded. Even partial collapses could be a catastrophe for millions of people worldwide.
You might think that since glaciers are gradually built up by years of accumulating snow, they melt gradually also. That seems not to be entirely the case-they may disintegrate in sudden bursts of ice ejected into the ocean that raise sea level even without the ice melting. This phenomenon is presently one of the biggest uncertainties about the future of climate change. As the ice at the margins of ice sheets like Greenland and West Antarctica melt, it's like pulling the plug on the river-like outlet glaciers that feed the bay. The ice accelerates toward the sea since the forces holding it back have eased, dropping more and more icebergs into the ocean in the process, and raising sea level as ice once locked on land is now dumped in the drink. In Greenland, between 1990 and 2005, the satellite-measured ice flow rate doubled by one measure and tripled by another.
Scientists have only recently realized that in addition to the edge erosion factor, glacial lakes formed from meltwater on the surface that drain to the base of the glacier through cracks called moulins are playing an unexpected role. As the surface lakes drain to the subsurface where the pressure keeps them from freezing, they act as lubricants that gradually free the glacier from its deep bedrock anchors.
And it's not inconceivable the ice sheets could retreat. Scientists have found DNA from beetles, butterflies, and conifers beneath Greenland's ice sheet showing that 400,000 years ago, its ice sheet was much smaller. Because this science is so new, none of the climate models used by existing IPCC reports take into account the dynamics of ice streams or an accurate representation of the bottom of an ice sheet. Thus, the potential contribution of polar ice sheets to sea-level rise is one of the biggest remaining uncertainties in climate science.
There are some aspects of climate change to which the word "cockamamie" can be validly applied-and these have to do with a topic called "geoengineering". Geoengineering is the deliberate large-scale modification of Earth's environment. Though some forms of geoengineering have great potential, others fall more into the category of "last resort".
Carbon capture and sequestration schemes fall in the first category, and include scrubbing carbon from the exhaust pipes of power plants or directly from the atmosphere, then injecting it into rock layers deep underneath the Earth's surface. In some cases this would be into the very same fossil fuel reservoirs we got the carbon from in the first place. On the more farfetched side are schemes like blocking sunlight by deploying millions of small mirrors into space. Ships could also shoot streams of water into the air to enhance oceanic cloud cover, a strategy called marine cloud brightening. Scientists are also looking at creating special tropical soils called terra preta, which are self-sustaining fertile soils (unlike most tropical soils, which are very poor). Terra preta were originally discovered by ancient native tribes and have the unusual side-effect of trapping carbon. Still others have suggested diverting Arctic rivers to prevent warmer fresh water from reaching the ocean, or even tethering icebergs to keep them from drifting into warmer waters where they will melt faster. Though melting icebergs won't change sea level, keeping the Arctic cold is important for avoiding melting permafrost, which could itself release tons of carbon dioxide and methane into the atmosphere.
Another geoengineering technique, iron fertilization, would take advantage of iron's status as the limiting factor on growth of ocean phytoplankton. Dumping lots of iron in the ocean fertilizes these organisms, which take up carbon dioxide as they grow. When they die, the carbon in their skeletons falls to the sea floor and is effectively removed from the atmosphere. But it comes at a price: iron also helps acidify the ocean, which is itself already a huge cause for concern. Another proposal involves releasing sulfate particles into the stratosphere, either with fighter jets or heavy naval artillery to replicate the cooling effects of a volcanic eruption.
Some of these seem wildly impractical and don't get at the root causes of climate change. Some could have unintended, disastrous consequences. For instance, if lots of carbon dioxide escaped from an underground reservoir at once, it could potentially suffocate any people or animals nearby. And on top of all of this is an ethical question: Who decides for all humanity to deliberately change the climate?