climate change

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Earth’s climate is vulnerable to potential climate catastrophes that could threaten the longevity of civilization. Continued increases in greenhouse gas forcing could lead to the collapse of major ice sheets, which would cause catastrophic sea level rise and could cause the oceanic thermohaline circulation to halt. Further warming could cause the heat stress index to exceed survival limits, inducing hyperthermia in humans and other mammals. Even more extreme warming could shift Earth into a runaway greenhouse regime that would lead to the loss of all oceans, and the end of all life.

Geoengineering refers to the large-scale use of technology to alter Earth’s global climate, and geoengineering has been suggested as a way to ameliorate contemporary climate change. Addressing these immediate climate challenges through a combined strategy of adaptation, mitigation, and (if needed) geoengineering is a critical issue facing us today. Whether or not we decide to engage in geoengineering today, we must still devise a long-term strategy to address our changing climate.

But in the longer-term, could we also use geoengineering techniques to increase the size of the polar ice caps? In a paper published in a special issue of the journal Futures, I raise the question, “Should we geoengineer larger ice caps?” By doing so, the global average temperature of Earth could be lowered from its current state to a new stable regime with much larger ice caps. Earth has experienced shifts in ice coverage in its past, and a prolonged program of geoengineering–say, lasting a thousand years or more–could allow us to permanently shift the energy balance of Earth. More ice at the poles increases the amount of sunlight reflected back to space, leading to cooler temperatures.

Of course, the unfortunate side effects of this idea would be mass migration of populations near the poles, shifts in global agricultural zones, and a required commitment of millenia in order to avoid undesired side-effects. Human civilization today probably lacks the fortitude to embark on such a long-term goal. Nevertheless, thinking about the long-term management of our planetary system helps us realize that we have already entered the epoch of the Anthropocene. Our civilization itself is fundamentally intertwined with our global climate, and we should allow humility, rather than hubris, guide decisions to control our environment.

Geologic records over the past million years indicate cycles in the extent of Earth’s surface covered by ice. These ice age cycles are a result of variations in Earth’s orbital geometry, but it is unclear how these variations will continue in the presence of significant human emissions. In a paper published in the Journal of Advances in Modeling Earth Systems, I develop a simple climate model to demonstrate the potential for human-induced climate change to damp out variations in ice coverage, which suggests that human actions today could have long-lasting impacts into the future.

Long-term patterns in Earth’s climate show glacial cycles that correspond to variations in Earth’s orbital geometry and affect the overall amount of sunlight that the planet receives. Known as “Milankovitch cycles”, these variations are observed in geologic reconstructions of temperature and isotopes to show periodic changes every 23,000, 41,000 and 100,000 years. The first two of these correspond directly to changes in Earth’s tilt (i.e. obliquity) and wobble (i.e. precession), but the longer 100,000 year variations in orbit seem too weak in magnitude to drive the strong climate signals we observe.

One solution to this problem is that the climate system itself amplifies these small changes to create more noticeable periodic signals. These amplification mechanisms could be the large thermal intertia of the oceans, the vast energy required to move giant ice sheets, or long-term cycles in greenhouse gases such as carbon dioxide and methane. Any combination of mechanisms such as these could magnify small changes in sunlight from Milankovitch cycles and create the dominant 100,000 year cycle in ice coverage seen in the geologic record.

Studying this problem has proven to be challenging because of the long time scales involved. Most contemporary climate models are focused on patterns of climate on Earth today and in the near future of a few hundred years from now, but few modelers have focused their attention on the more distant future of climate. In my paper I develop a simple climate model that uses stochastic (i.e. randomly generated) forcing to achieve a state of resonance that displays a 100,000 year cycle in ice coverage. The model is an idealization of the more complicated Earth system and provides a tool for exploring the behavior of climate over these long time scales.

Calculations with this model show that the influence of human emissions into the atmosphere can affect the presence of the ice age cycles, either by damping the magnitude of changes or by ceasing the cycles altogether. The simplified calculations here cannot predict exactly when this should occur, but this study points toward the existence of a threshold beyond which ice age cycles may cease as a result of human emissions.

The future of Earth’s climate is becoming increasingly marked by the presence of human activity. Depending on the course of events over the next few hundred years, we may find that the damping or cessation of ice age cycles is yet another indicator of the dawning of the age of the anthropocene.

Evidence for anthropogenic climate change continues to build, but even some academics retain a skeptical attitude toward the radical claim that humans are significantly changing the climate of Earth. In a recent article published in Science and Engineering Ethics, on which I am co-author, we discuss several issues raised by climate change skeptics.

The first issue we address is an observed decline in global temperature from 1943 to 1975, which resulted from an increase in emission of sulfur aerosol from industrialization that scattered away sunlight and cooled the surface. The second issue is the use of ice core data to reconstruct ancient climate data, which requires the careful extraction and characterization of air bubbles trapped within the ice. The third issue concerns the role of clouds in climate change, which indeed is one of the greatest sources of uncertainty regarding climate change. We also discuss other key uncertainties, such as the sensitivity of surface temperature to greenhouse gas concentration, the contribution of glacier melt to sea level rise, and the regional effects of climate change.

What society should do about climate change is fundamentally an ethical question. The answer to this question depends on our ethics–that is, on what we think is right and wrong or good and bad. Some skeptics might argue that we should not address climate change because of the expense involved in mitigation or adaptation, but the direct and indirect damages from climate change will be costly to businesses, governments, and individuals. Based on these cost considerations as well as a regard for human civilization in the future, a strong ethical argument can be made that society should prioritize reducing greenhouse gas emissions to reduce how much the climate will change.