Climate History

Using ice core data, we can deterimine global temperatures and atmospheric composition going back 430,000 years. This is done via analysis of oxygen and carbon isotopes found in air bubbles in the ice cores. Here is the graph of temperature and carbon dioxide over the past several hundred thousand years:

Temperature and CO₂ data derived from the Vostok, Antarctica ice core. The broader dips are ice ages and the narrower peaks are interglacial periods.
Source: http://maps.grida.no/library/files/26.jpg

What does this graph show us?

• Carbon dioxide concentrations dropped to 180 ppm during the coldest periods and reached a maximum of 300 ppm in the warmest periods. Current CO₂ concentrations are at about 380 ppm.
• Carbon dioxide and global mean temperature are highly correlated.
• There are clear cycles of ice ages (the broad dips) and interglacial periods (the narrow peaks).
• We are currently in an interglacial (warm) period.
• "The Long Summer": The current interglacial period (~10,000 years; the final peak on the bottom graph) appears to have lasted much longer than previous ones.
• This graph tells us nothing about short term trends (i.e. on the century time scale of recent global warming).

Why is this graph important?

We all have heard that "correlation does not imply causation." However, with such a high correlation, it's likely safe to assume that one variable likely modulates the other, whether through direct causation or not. On long-term time scales, it does not seem likely that carbon dioxide could modulate temperature in such a dominant manner. Thus, it is likely that temperature would modulate carbon dioxide over time, which makes sense: global temperature, which changes primarily due to solar cycles (see below), modulates biological activity and ocean storage of CO₂ (a warmer ocean retains less carbon dioxide, so as temperatures rise (fall), the oceans "exhale" ("inhale") carbon dioxide). However, while carbon dioxide changes as temperature changes, CO₂ has a positive feedback (it induces some warming, which causes more water vapor to enter the atmosphere, which further increases the greenhouse effect) that allows the climate system to undergo dramatic temperature swings as the solar cycles and other forcings change the Earth's temperature.

This positive feedback is the primary reason why the current emission of CO₂ is such a concern. It appears that our climate past has undergone dramatic climate shifts in short periods of time due at least in some part to atmospheric carbon dioxide. Paleoclimate models have a difficult time reproducing the recent ice age cycles without including this carbon dioxide feedback, implying that carbon dioxide is indeed important in our climate. This is certainly cause for alarm given how much CO₂ we are emitting today.

The Earth (almost) goes round

The Milankovich cycles: The primary forcing for the Earth's mean temperature and ice age cycles is the set of three solar cycles known as the Milankovich cycles. They are caused by changes in the Earth's orbit around the sun. These three cycles--eccentricity, obliquity, and precession--all deal with regular cycles in the tilt and wobble of the Earth on its axis, as well as the distance of the Earth from the sun. These cycles occur on 22,000, 41,000, and 100,000 year intervals, and the cycle of ice ages and interglacial periods observed over the past 500,000 years (discussed at top) follow very closely with these solar cycles

Averting an ice age?

Based upon these cycles, a recent theory has been proposed by Dr. William Ruddiman regarding the onset of the human impact on the Earth system:

Around 8000 years ago, the Earth's climate broke from the well-known Milankovich cycles. After reaching the expected thermal maximum 11,000 years ago, carbon dioxide levels and temperatures began to drop as expected--and then stopped. This was around the time that human influence began to grow: rapid proliferation of large-scale agriculture and deforestation that began major increases in emissions of methane and carbon dioxide. Furthermore, the onset of large-scale human influence represents the only known major shift in the Earth system during this period. Overall, during a period of expected cooling (perhaps into another ice age following the observed cycles), the Earth reached a period of climate stability (8000 years ago through today) known as the "Long Summer."

The fact is that for hundreds of thousands of years, it appears that carbon dioxide concentrations follow temperature cycles (governed by the Milankovich Cycles). With the introduction of large-scale human influence--i.e. moving beyond simple hunter/gatherer society to one of mass deforestation and farming, where one human being is capable of altering large fields of land--we began an unintended experiment of the reverse process: can atmospheric gases regulate temperature? What is the result of a disequilibrium in which the CO₂, instead of the temperature, is changed dramatically? It's reasonable to assume that if its impact was small, then the temperature trend would have continued to follow the natural cycle regardless of changes in CO₂. But it hasn't.

Is there precedent for such an "experiment" in the past?

There most likely is, perhaps via volcanic activity or other mechanisms. However, 2 problems arise in searching for such an event:

1. An event such as a volcano emits many other gases (e.g. sulfates) that blur the effects of the component gases such as CO₂
2. Given that CO₂ has an atmospheric lifetime of only 5-150 years, this means that the lag time between emission and temperature feedback is so short that it's impossible to distinguish in a long-term record (such as the graph at the top of the page) whether a given temperature shift/trend is caused by changes in gas concentration or if it's the other way around.

This leads to the next question...

If CO₂ has been emitted in the past, why is it suddenly important now?

It's important now for one simple reason: The rate of climate change is what matters most. Given the positive feedback from carbon dioxide, such massive emissions of this gas over such a short time would imply significant climate shifts: A 2C change over a few thousand years can be adapted to by animal and plant species. But species cannot adapt if the climate changes too quickly; a 2C change over one hundred years or less is widely considered to be far too short of a timeframe to adapt. Given that warming has been accelerating over the past half-century, concern is warranted, as global warming is trending towards climate change rates that are potentially dangerous for ecosystem survival. Humans can pull up a blanket; animals and plants cannot.

Recent changes: Humans undoubtedly can change the climate

Here are some recent studies/information on human activity over the past century:

• In the past 50 years, the world has lost 1/4 of its topsoil and 1/3 of its forest cover (Nat Cap 4).
• Aerosols (see Global Dimming)
• Sulfate aerosols, which were released into the atmosphere in large quantities before the 1970s in low-quality coal burning powerplants, are excellent at reflecting sunlight
• Jet contrails are shown to have a substantial (1-2F) influence on the daily temperature range (difference between nighttime low and daytime high)
• We may be running out of oil: This alone should be a major common-sense concern, given that it took natural processes millions and millions of years to create those oil beds from the decomposition of dead matter, and we have now dug up and burned these beds in a matter of 200-300 years. To assume that such an action would have no major impact on the Earth system is willful ignorance.

Overall

We do not understand many of the details of Earth's climate history or the Earth system and how it responds to different forcings.. However, we do know that our history is filled with major climate shifts that may be linked with major shifts in atmospheric gas concentrations. Thus, the common sense response is: "Hey, let's not mess with a system that we dont really understand but that is essential to our very existence." And based on the brief climate history of modern humans, who are the first animals to single-handedly and voluntarily cause significant changes in the Earth system, we have already begun to do what common sense tells us not to.

How might our future climate look?

Check out Climate Models.

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