Interglacials and the Future

Doug Hoffman, a thoughtful global warming skeptic, writes on his blog The Resilient Earth:

All of this wicked weather comes with the often repeated warning that weather is not climate. So what is going on with the climate? It looks like a combination of El Niño and the multi-decadal oscillations in the Pacific and Atlantic are conspiring to cause a temperature downturn world wide. In the near term we are in for the usual multi-decadal variations in hot and cold. Prof Anastasios Tsonis, head of the University of Wisconsin Atmospheric Sciences Group, has recently shown that these MDOs move together in a synchronized way across the globe, abruptly flipping the world’s climate from a ‘warm mode’ to a ‘cold mode’ and back again in 20 to 30-year cycles.

“They amount to massive rearrangements in the dominant patterns of the weather,” Tsonis explained, ‘and their shifts explain all the major changes in world temperatures during the 20th and 21st Centuries. We have such a change now and can therefore expect 20 or 30 years of cooler temperatures.” The bigger question is, does the current cooling trend indicate the start of the next transition from interglacial back to glacial conditions—are we headed back into an ice age?

The general term “ice age” or, more precisely, “glacial age” denotes a geological period of long-term reduction in Earth's surface and atmospheric temperature, resulting in an expansion of continental ice sheets, polar ice sheets and mountain glaciers. For the past several million years, Earth has been locked in an ice age called the Pleistocene. Glacial periods come and go with remarkable regularity, lasting 100,000 years or so and then transitioning to a much warmer interglacial period for 10,000 to 15,000 years. The last such glacial period finally ended about 11,000 years ago (see “Variations in the Earth's Orbit: Pacemaker of the Ice Ages”).

If the current interglacial, optimistically designated the Holocene epoch, follows the previous cycles there could be several thousand years of temperate climate left. But that is just working with average behavior. There have been shorter interglacials and much longer ones as well. Subtle shifts in Earth's orbit around the Sun moderates the amount of energy entering the Earth's climate system. These variations in Earth's orbital and rotational configuration are known to trigger or end glacial periods, but other factors come into play as well.

Variations in solar output occur on much shorter, decadal time-scales than glacial-interglacial climate oscillations. Atmospheric carbon dioxide concentrations also correlate nearly perfectly with Ice Age cycles—but CO2 is thought to be a positive feedback, not the trigger. Other feedback mechanisms—the amount of airborne dust, changes in albedo, shifting ocean currents, etc.—complicate the interplay of environment and solar radiation. When the combination of influences are right Earth's climate undergoes dramatic changes.

There are also longer cycles seen within warm interglacial periods. Perhaps caused by longer term variation in solar output, these cycles can span for centuries. Since people have been around to directly observe them these alternating periods of warm and cold have been given names: The Little Ice Age, the Medieval Warm Period and so on. While Earth has indisputably been in a warming trend for the past 150 years or so, our planet may well enter a new cooling phase that could last for centuries.

Most likely, a thousand years from now the climate will have resumed warming. This means that glaciers and the great ice sheets of Greenland and Antarctica will continue to melt, raising sea-levels around the world by several more meters. Many coastal areas will be flooded and humanity will be called on to do what it does best, adapt. My forecast for 3010 AD is for slightly warmer temperatures, moderately higher sea-levels an a slightly more active hydrological cycle (more rain). You may have to move your beachfront cottage, but other than that things will be quite comfortable.

Contrary to the dire warnings that rising sea-levels constitute a climate crisis, rising sea-levels during an interglacial are perfectly natural—an interglacial is one long stretch of glacial melting (see “Ice Core Evidence for Extensive Melting of the Greenland Ice Sheet in the Last Interglacial”). The Eemian interglacial period, the second-to-latest interglacial period of the Pleistocene Ice Age Ice, reached its height around 125,000 years ago. Sea-level at that peak was probably 4-6 m (12-20 ft) higher than today (see “Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise”). Much of this extra water came from Greenland but some also came from Antarctica. When the glaciers and ice caps stop melting and ice begins to accumulate again it is a sign that Earth is beginning another slow descent into ice age conditions.