A big problem with predicting responses to global climate change (or other environmental changes) is that they are nonlinear and thus disproportionate. Sometimes large changes can have relatively small responses, while in other cases small changes can have disproportionately large impacts.

Responses to environmental change are sometimes characterized by amplifiers—phenomena that reinforce or exaggerate the effects of the change. For example, if coastal land is subsiding, this amplifies the effects of sea level rise. Or, when warming results in permafrost thawing, this releases methane, a heat-trapping greenhouse gas, this leads to further warming. However, there are also filters—phenomena that resist, offset, or diminish the effects of the change. For instance, if coastal land is tectonically or isostatically uplifting, this can offset or even eliminate effects of sea level rise with respect to coastal submergence. Or, if warming results in increased cloud cover, which reflects more radiation, this counteracts the warming.

Marshes in the Trinity River delta, Texas, part of the Galveston Bay complex (Google Earth image). Here subsidence due to a combination of natural compaction and human activity (extraction of oil, gas, and water) is resulting in subsidence of the wetlands, which amplifies effects of rising sea level. 


I was reminded of this by today’s news ( of confirmation of an amplifier effect of global warming—water vapor increase in the upper troposphere. Because H2O is a greenhouse gas, and cloud formation is not an issue in the upper troposphere, this is a clear amplifier effect. The summary of the news story: “A new study . . . confirms rising levels of water vapor in the upper troposphere -- a key amplifier of global warming -- will intensify climate change impacts over the next decades. The new study is the first to show that increased water vapor concentrations in the atmosphere are a direct result of human activities.”

The responses to the (proven and undebatable) increase in the global concentration of carbon dioxide and other greenhouse gases include both amplifier and filter effects, as we talk about quite a bit in my GEO 130 (Earth’s Physical Environment) class. If it were not for some of the filter effects (i.e., if the physics of greenhouse gases were the only thing affecting temperature), it would already be hotter than it is (and it is getting hotter, globally). On the other hand, right now the weight of the evidence suggests that the amplifier effects, such as the tropospheric water vapor, and the fact that land and ocean surfaces bared by melting ice absorb more radiation than ice, are winning.

The amplifier and filter concept also applies to geomorphological, ecological, and other (including economic and political) responses to climate change. The climate changes themselves can be amplified or filtered, and the responses of, for instance, landforms or vegetation to the net climate change can themselves be amplified or filtered. Thus you could have several layers of amplification ratcheting up the effects of change, several layers of filtering diminishing or obscuring those effects, or some combination of amplifiers and filters.

I examined this in the context of how Kentucky rivers have responded to climate change in the last couple of million years (Phillips, 2010). Another piece discusses amplifiers and filters in more detail with respect to how landforms and landscapes respond to all kinds of disturbances (Phillips, 2009).

Phillips, J.D. 2010.  Amplifiers, filters, and the response of Kentucky rivers to climate change. Climatic Change 103: 571-595.

Phillips, J.D.  2009.  Changes, perturbations, and responses in geomorphic systems.  Progress in Physical Geography 33: 17-30.