Ice Age Cycles and Climate Change

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.

Geoengineering Double Catastrophe

If geoengineering by injection of aerosol particles into the stratosphere is deployed, then the occurrence of a global catastrophe could cause intermittency in geoengineering and lead to total damages far greater than if either situation occurred in isolation. While the outcomes of the double catastrophe are difficult to predict, plausible worst-case scenarios include human extinction. In a paper published this month in the journal Environment Systems & Decisions, on which I am a co-author, we develop this double catastrophe scenario, which strengthens arguments for greenhouse gas emissions reductions and demonstrates the value of integrative, systems-based global catastrophic risk analysis.

Global catastrophic risks are risks of events that would significantly harm or even destroy humanity at the global scale, such as climate change, nuclear war, and pandemics. To date, most research on global catastrophes analyzes one risk at a time. A better approach uses systems analysis to capture the many important interactions between risks. This paper analyzes a global catastrophe scenario involving climate change, geoengineering, and another catastrophe. We call the scenario “double catastrophe”.

The rising temperatures of global climate change pose great risks to humanity and ecosystems. Climate change can be slowed by reducing emissions of greenhouse gases like carbon dioxide and methane. But humanity has been struggling to reduce emissions. One alternative is geoengineering, the intentional manipulation of Earth systems. The most promising geoengineering option may be stratospheric geoengineering, in which aerosol particles are put into the stratosphere. The particles block sunlight, lowering temperatures on Earth’s surface.

One problem with stratospheric geoengineering, known as intermittency, is that the particles must be continuously replaced in the stratosphere. If they’re not, then in a few years they fall out, and temperatures rapidly rise back to where they would have been without the geoengineering. The rapid temperature increase would be very damaging to society. Because of this, society is unlikely to let intermittency occur–unless some other catastrophe occurs, knocking out society’s ability to continue the geoengineering. Then, the rapid temperature increase hits a population already vulnerable from the initial catastrophe. This double catastrophe could be a major global catastrophe.

Because of how damaging global catastrophes would be to human civilization, decision making is often oriented towards minimizing the risk of global catastrophe. Stratospheric geoengineering can prevent global catastrophe from climate change alone, but it can also lead to global catastrophe from the double catastrophe scenario. If global catastrophe is more likely from climate change alone, then society should decide to implement stratospheric geoengineering. Otherwise, society is better off without stratospheric geoengineering. This assumes (among other things) that the goal should be minimizing global catastrophic risk and that stratospheric geoengineering is the best form of geoengineering.