Made by God, immutable, perfect and constant, our world has always been the way it is now and will always be the same. Perhaps it is a human desire for order and predictability that is at the root of such traditional beliefs. But pesky facts get in the way. Fossils provide evidence for creatures that must have lived long ago and gone extinct. Rock layers and mountains ranges show a history of constant upheaval.
The static world is replaced by one where ever-present change is the grinding of tectonic plates, the random mutations of Darwinian evolution, the expansion of the Big Bang and the slow ebb and flow of glaciers over the land. This is the world, predictable and gradual, that was contemporary science when I was in school and remains the dominant mind-set for many today.
In the late 1980′s scientists started recovering the history of the planet laid down in polar ice. There are few scientific observations that have so dramatically changed how we look at the world. I still recall the first time I saw the deep-time series data from the Antarctica ice. The cores gave us an unprecedented, highly resolved, long-term climate history. There, laid before me was a past unlike anything I had imagined. Rather than textbook ever-so-gradual changes over millennial time scales, there were instead abrupt warming events punctuated about every hundred thousand years , with large amounts of rapidly fluctuating climate “noise” in between.
So much for the “gradual change” model of our earth’s climate. The idea that glaciers gradually advanced over the continents, reached a maximum point and then just as gradually retreated is clearly wrong. Instead, the rapid temperature increases followed by the more gradual temperature drops suggest a kind of “relaxation oscillator” mechanism at work where the climate switches between cooling and warming states.
The major features of the temperature data can be correlated with variations in solar flux associated with periodic changes in the earth’s orbit and tilt of the poles, known as Milankovitch cycles. However, the climate forcing due to orbital changes is the result of the sum of symmetric functions, and as a result, qualitatively looks the same if time were to run either forward or backward. Not so for the actual climate data. The Milankovitch oscillations appear more like triggers that release pent-up climate tensions. The climate has its own dynamic behavior that is not merely a linear response to external forcing. The rapid temperature rises suggest powerful feedback mechanisms that reinforce the switch to warmer climate. The heat trapping CO2 concentration, which mimics the temperature data, could certainly be one of these mechanisms. More solar warming as reflective ice sheets recede, leaving behind heat absorbing dark water and land, must also be important.
The violence and the chaos associated with finer structure in the temperature data is equally surprising, perhaps best seen in some of the ice core data from Greenland.
Here we see a comparison of the Antarctica and Greenland data for the more recent past extending back about 50,000 years. The temperature, deduced from oxygen isotope ratios, is inherently a local measurement and subject to local weather patterns and difficult calibration issues, so the absolute magnitude of the changes in the northern and southern data is probably less significant than the relative level of fluctuations. The Greenland data show very rapid and very large fluctuations in the the temperature – much more so than the Antarctic data, although the basic warming and cooling trends are seen in both. This suggests that the driver of rapid climate change is primarily located in the northern hemisphere, where the polar region contains much more open water than in Antarctica. Rapidly disintegrating ice sheets over open ocean could result in changes in atmospheric or oceanic circulation patterns and provide strong positive feedback for temperature change when the ocean goes from a white reflector to dark absorber of solar radiation – or vice versa.
The Younger Dryas is a period between about 11,000 and 13,000 years ago, during which the climate, coming out of the last great ice age, again rapidly fell back into an ice age for a couple of thousand years, and then even more abruptly warmed to near modern temperatures. These events have been studied extensively in an attempt to understand what was going on during the last great abrupt climate changes. The high temporal resolution of the Greenland ice cores showed that the warming event occurred on a time scale of just a few years. Attempting to fully understand this event is still an area of active research and paleoclimate modeling.
Current human-induced atmospheric emissions and land use changes are propelling the climate into a world that the last million years of ice core data never experienced. Already in an un-glaciated world, we are now melting the polar ice that has survived for eons. As the melt regions move poleward, carbon that has been frozen in place for the entire ice core record becomes available to the atmosphere, as methane and CO2 evaporate from the tundra. The pent-up climate-changing potential is unknown and almost unknowable, because the best historic guide we have, the ice cores, have never recorded an event similar to the grand experiment humankind is perpetrating on the planet. The lesson that these records teach is that warming events happen very rapidly — out of all proportion to the trigger climate forcing event. On the graph of the CO2 levels from the Antarctic ice core, I have added the modern contribution. The vertical line extending to 390 ppm is an unprecedented dramatic climate forcing event on its own, even not including whatever feedback effects might amplify a climate mode-shift. We are playing with fire!
Climate gradualism should have died when the ice core histories became available, but our cultural history has its own gradualism built into it. Changing our collective world view may take decades longer than it does for a major climate circulation mode-switch. Time will tell, but time is not on our side as we still empty our coal and oil reserves into the sky.