1. Natural Causes of Climate Change
and
Past Climates (Paleoclimatology)

2. Geologic Time and Human Evolution

3. Geologic Timescale
Red and blue areas indicate hot (Hot House) and cold (Ice House) periods

4. Geologic Time Scale Relative to One Year

5. Major Human Events on a Relative Time Scale of One Year
*Starting time to present

6. The Anthopocene Epoch
The Anthopocene is a new Epoch characterized by human-caused major global changes that have altered the Earth in fundamental ways.
It starts at the beginning of the Industrial Revolution in 1751.
We are no longer in the Holocene.

7. Mass Extinctions
The K/T extinction was due to a large impact that radically changed the climate.
The other extinctions appear to be due to natural climate changes.
The greatest mass extinction (P/T) was due to a climate change from an Ice House to a Hot House.

8. Causes of Climate Change
Abundance of Greenhouse Gases
Major Volcanic Eruptions
Large Asteroid or Comet Impact
Change in Sun’s Irradiance
Change in Ocean Circulation
Continental Drift
Change in Earth’s Motions
This may seem like a bewildering array of processes that cause climate changes, and that almost any one or some combination of these processes could be the result of the present global warming. However, only two processes operating today cause warming fast enough to result in the present global warming; 1) increase in greenhouse gas abundance, and 2) increase in solar irradiance. While increased solar irradiance increases the global temperature, it is not strong enough to cause the entire present rapid temperature increase. It is only responsible for about 3% of the warming. The rapid increase in greenhouse gases is the main cause of current global warming.
Only two causes of climate change can operate on time scales short enough to account for today’s rapid temperature rise; 1) increase of greenhouse gases and 2) increase in solar irradiance. Although large volcanic eruptions work on a very short time scale, they cool the climate, not warm it. They can warm the climate if there are numerous gigantic eruptions that emit little ash and a lot of CO2. However, that is not happening now. A rapid change in the present ocean thermohaline circulation would also cool the climate in parts of the high latitude northern hemisphere. Large asteroid or comet impacts can also change the climate in a very short time, but that is obviously not happening today. The other causes, such as continental drift or orbital variations, operate on too long a time scale to account for the current rapid increase in temperature.
Red = main cause of current global warming

9. Past Oxygen and CO2 Abundances
The amount of greenhouse gases in the atmosphere is probably the most obvious cause of climate change. The most important greenhouse gas is carbon dioxide (CO2) because it is the most abundant. However, methane (CH4) may have played a major role in some climate changes. There can be natural increases or decreases in these gases depending on the degree of biological activity and other factors. Figure 4.2 shows the estimated variation of oxygen (O2) and CO2 over the Phanerozoic Eon. The scale of these changes is still uncertain but their existence is well established. Although the abundance of O2 is extremely important to evolution and a good indicator of biological activity, it is not a greenhouse gas and, therefore, of no consequence to climate change. The balance between CO2 and O2 is the result of the biologic cycle, which depends on the interaction of these gases with life and its waste products. Oxygen is produced by photosynthesis of plants. It is taken up by animals, oxidized and emitted as carbon dioxide. The CO2 is then taken up by plant photosynthesis to complete the process.
If biological productivity is low, then CO2 is not taken up as efficiently and the CO2 content of the atmosphere increases while the O2 abundance decreases. Conversely, if the productivity is high then CO2 declines and O2 increases. For example, the huge production and deposition of organic matter during the Carboniferous Period (the name derives from the huge carbon deposits) probably led to the large increase of O2 and corresponding decrease of CO2 (See Figure 4.2).
Two other processes linked to plate tectonics can also change the level of CO2 in the atmosphere over tens of millions of years. One is the atmospheric input from volcanic eruptions and the other is the weathering of silicate rocks. Beside water vapor, the main gas emitted by volcanic eruptions is carbon dioxide. Variations in the intensity and frequency of volcanism with time will change the CO2 level. Variations in the extent of silicate rock exposure over time can also affect the abundance of atmospheric carbon dioxide. Large areas of exposure will deplete the abundance more than small areas. The combined effects of biological productivity, volcanic activity and silicate rock weathering can change the various proportions of CO2 and O2 in the atmosphere.
There is also a strong possibility that methane (CH4) locked up in ocean sediments has been released catastrophically to raise the temperature to high levels in short periods of time. For example, stable carbon isotope ratios (13C/12C) from ocean sediment cores indicate that about 1,200 to 2,000 billion metric tons of methane were released into the atmosphere about 55 million years ago. The gas was released from methane-rich ocean sediments probably by global warming that pushed the ocean-atmosphere system past a critical threshold. This powerful greenhouse gas probably caused the extreme warming at the end of the Paleocene Epoch (see Chapter 5).
Increases or decreases in greenhouse gases can thus be long-term or very short-term. Currently the increase in greenhouse gases is happening very rapidly, even on a human time scale. There is even a possibility that the current rapid increase in temperature could trigger a rapid release of methane and a devastating rise in temperature.

10. Major Volcanic Eruptions
Large volcanic eruptions can cool the climate for a few years by injecting ash into the stratosphere to reflect the Sun’s radiation back to space.
Enormous eruptions over long periods can emit large amounts of CO2 to warm the climate.

11. Large Asteroid or Comet Impacts
Large impacts cool the climate by injecting dust into the stratosphere to reflect sunlight back to space.
If the impact is in limestone a large impact will first cool the climate and then heat it up by releasing large amount of CO2 from the limestone.

12. Changes in the Ocean’s Thermohaline Circulation Can Change the Climate

13. Continental Drift due to Plate Tectonics can Change Climate over Millions of Years

14. Changes in Earth’s Motions can Change the Climate

15. Changes in the Sun’s Irradiance

16. Let me make note of the assertion that the world could be headed into colder times because of changes on the sun, because that misconception has been spread widely.
Solar irradiance has been measured since the late 1970s, and the solar irradiance remains at or near a prolonged solar minimum, which is deeper than the prior measured minima. This is data of Frohlich and Lean through the end of September.
These solar irradiance variations do not have any known relation with the shorter period oscillations of Pacific Ocean temperature. In a few moments I show quantitatively that the effect of the sun is not negligible on longer time scales (the time scale of the year solar cycle and longer time scales), but it is much smaller than the climate forcing due to human-made greenhouse gases.
Soar irradiance through September Reference: Fröhlich, C. and J. Lean, Astron. Astrophys. Rev., 12, pp ,

17. Solar Irradiance, Temperature, and Atmospheric CO2

18. Solar Irradiance, Temperature, and Human-Caused CO2 Emissions

19. Conclusions
Only two causes can operate on time scales short enough to account for today’s rapid warming: 1) increase in solar irradiance, or 2) increase in greenhouse gases.
The increase in solar irradiance is not enough to account for the present warming and its rapid rise.
The increase in greenhouse gases must be the cause of global warming.
This is consistent with the observed rapid rise in both greenhouse gases and their emission by humans.

20. Generalized Climates for the Past 3 Billion Years
The diagram shows only the dominant climatic regime in Earth’s history. That history has been dominated by Hot Houses with temperatures well above today’s and little or no ice on Earth. Ice Houses consist of alternating glacial and interglacial periods. They have been relatively rare throughout most of geologic history. We are now in an interglacial period of an Ice House. Not shown are limited cold periods in some Hot Houses and limited warm periods in some Ice Houses. There have only been three significant Ice Houses in the past 540 million years. We are currently in the third.

21. Cenozoic Era End of Cretaceous (65 My BP) Present Day
The dominant climate forcing over at least the first half of the Cenozoic must have been CO2, because CO2 was as much as 1000 ppm, causing a forcing of more than 10 W/m2 relative to glacial periods. Other candidate forcings are an order of magnitude smaller.
The large CO2 changes are no surprise as the volcanic source and weathering sink of CO2 are not in general balanced.
At the beginning of the Cenozoic India was still south of the equator, moving north rapidly at 20 cm per year, plowing through the Tethys Ocean that had long been the depocenter for carbon-rich sediments from major rivers of the world.

22. Climate Change During Past 180 Million Years
Changes in the Earth’s temperature through time. Red areas are warmer and dark blue areas are colder. The temperatures are referenced to the average global temperature of 14° C. (Left) The top diagram spans the period from about the lower Mesozoic Era to the present shown at the bottom of the previous slide. The two bottom diagrams are successive enlargements of the present “Ice House” during the Pleistocene and Holocene Epochs. (Right) The top diagram shows the end of the last ice age and the present interglacial period (Holocene). Each of the enlargements show successively more detail including current global warming. All of human evolution has occurred in an Ice House and all of civilization has occurred in the present interglacial period (the Holocene). Modified from Cook et al., 1991.

23. The Paleocene-Eocene Thermal Maximum (PETM)
After the perturbations of the K/T impact, the climate in the Paleocene Epoch returned to hot conditions similar to the late Cretaceous. There was a brief exceptionally warm period at the end of the Paleocene about 55 million years ago called the Paleocene Eocene Thermal Maximum (PETM) shown in the slide. This event is important because it may have ominous implications for our current warming trend. Over a period of about 10,000 years at the end of the Paleocene Epoch, the Earth’s climate and oceans warmed (Zachos et al., 2003). The atmospheric surface temperatures rose between 5 and 10 C, and the deep-ocean and high-latitude surface waters soared by 8º to 10º C. Bottom water temperatures rose by about 4 to 5 C. In the tropical Pacific Ocean the sea surface temperatures rose about 4 to 5 C (Tripati and Elderfield, 2005). Temperature proxies (climate indicators) from sediment cores near the North Pole indicate a water temperature of about 24 C or the same as present-day subtropical ocean temperatures (Sluijs et al., 2006). Today the water temperature in the Arctic Ocean is about –2 C. Fifty-five million years ago the CO2 level was probably about ppm. Disturbingly, climate models for 55 million years ago do not come close to simulating such warm waters even with CO2 levels as high as 2000 ppm. During that time many deep-sea species of foraminifera (microscopic ocean animals) became extinct, and numerous mammalian orders appeared, including primates. Today methane and CO2 are starting to be released from melting permafrost and probably methane hydrates(see slides Arctic section). We may now be in a positive feedback similar to the PETM.
This extraordinary event coincided with a drastic decrease in the 13C/12C isotope ratio of carbon reserves (Norris and Rohl, 1999). It is probably explained by the addition of isotopically light CO2 to the ocean. The drastic increase in temperature at that time was probably due to the release of massive amounts of CO2 or methane. Alarmingly, today human activity is releasing greenhouse gases 30 times faster than they were being emitted then (Zachos et al., 2003). Three mechanisms have been proposed for this release. One is a sudden change in ocean circulation causing a catastrophic release of about 1,200 to 2,000 billion metric tons of carbon in the form of methane trapped in marine sediments. About 2/3 of this release was completed in a few thousand years at most. A second possibility is the release of very large quantities of methane from continental areas. Fifty-five million years ago the world was very dry and large sustained peat fires may have released enormous quantities of methane to the atmosphere causing the thermal maximum (Katz et al., 1999). A third possibility is that massive volcanism from newly discovered 55 million year old vents under the Atlantic Ocean disturbed huge amounts of buried methane and CO2 gas in marine sediments. The estimate is about 1500 billion metric tons of carbon equivalent released in a geologically very short period of time. This climate extreme lasted at least 100,000 years. Today there is about 14,000 billion tons of methane locked up in marine sediments that could be released by some triggering mechanism (probably ocean temperature increases). There are also huge quantities of methane locked up in northern peat lands and this gas is now beginning to be released in some localities.
When the continents migrated to a distribution similar to today, the climate began to change (Figure 5.1 and 5.2). After an early warm period in the early Eocene, the Earth began to cool off about 45 million years ago, but not uniformly (Figure 5.4). In both the Arctic and Antarctic at that time icebergs were present and CO2 levels were declining (Moran et al., 2006). They continued to drop until about 20 million years ago. There were two rapid cooling events one at the Eocene/Oligocene boundary about 32 million years ago and another at 15 million years ago. A sea level change occurred near the end of the Eocene and beginning of the Oligocene. This suggests that glaciers were forming in Antarctica at that time and recent studies indicate ice was forming in the Arctic in the Mid-Eocene about 45 million years ago (Moran et al., 2006). Unlike today there were forested areas in Antarctica as late as 25 million years ago.
There was further cooling about 12 to14 million years ago that initiated mountain glaciers in the northern hemisphere, and established a permanent ice sheet over east Antarctica. Finally, a sharp cooling between 5 and 6 million years ago produced a permanent ice sheet over all of Antarctica and established the current Ice House. Beneath a 0.5-meter debris cover in the ice-free Beacon Valley, part of the Dry Valley region of Antarctica, there is still relic ice greater than 8.1 million years old from the Miocene glacial epoch (Marchant et al., 2002).
The causes of the Tertiary general cooling are probably linked to continental drift that increased the amount of land at high northern latitudes. This made it easier for ice sheets to form, and resulted in various openings and closings of seaways changing the thermohaline circulation of the oceans. Also there were decreases in the CO2 levels.

24. Back to paleoclimate. It is useful to look at longer time scales.
Pleistocene climate oscillations are complex, including dynamics and interactions.
But the detailed information that we have on Pleistocene oscillations should not cause us to lose sight of the forest for the trees.
The surface albedo change in going from the ice-free state, such as existed 50 million years ago, to the depths of the ice age, or even snowball Earth conditions is entirely a feedback, which makes climate more sensitive.
Between the depths of the last ice age and deglaciation of Antarctica climate sensitivity to a specified greenhouse gas change is doubled to about 6 C for doubled CO2 because of the surface albedo feedback.

25. The Pliocene/Pleistocene “Ice House”
This figure shows the climate record of Lisiecki and Raymo (2005) [1] constructed by combining measurements from 57 globally distributed deep sea sediment cores. The measured quantity is oxygen isotope fractionation in benthic foraminifera, which serves as a proxy for the total global mass of glacial ice sheets.Lisiecki and Raymo constructed this record by first applying a computer aided process of adjusting individual "wiggles" in each sediment core to have the same alignment (i.e. wiggle matching). Then the resulting stacked record is orbitally tuned by adjusting the positions of peaks and valleys to fall at times consistent with an orbitally driven ice model (see: Milankovitch cycles). Both sets of these adjustments are constrained to be within known uncertainties on sedimentation rates and consistent with independently dated tie points (if any). Constructions of this kind are common, however they presume that ice sheets are orbitally driven, and hence data such as this can not be used in establishing the existence of such a relationship.The observed isotope variations are very similar in shape to the temperature variations recorded at Vostok, Antarctica during the 420 kyr for which that record exists. Hence the right hand scale of the figure was established by fitting the reported temperature variations at Vostok (Petit et al. 1999) to the observed isotope variations. As a result, this temperature scale should be regarded as approximate and its magnitude is only representative of Vostok changes. In particular, temperature changes at polar sites, such as Vostok, frequently exceed the changes observed in the tropics or in the global average. A horizontal line at 0 °C indicates modern temperatures (circa 1950). Labels are added to indicate regions where 100 kyr and 41 kyr cyclicity is observed. These periodicities match periodic changes in Earth's orbital eccentricity and obliquity respectively, and have been previously established by other studies (not relying on orbital tuning). For discussion of how such orbital changes might drive climate change, see Milankovitch cycles. Also shown are the approximate times when Hominids and Homo Sapiens first emerged. All of human evolution has occurred in an Ice House. Humans and proto-humans have never experienced Hot House conditions.

26. Summary: Cenozoic Era Dominant Forcing: Natural CO2
- The natural imbalance of CO2 between sources and sinks is about ppm/year or 100 ppm/million years
- Human-made rate today: ~2 ppm/year
- It takes humans 1 year to emit what nature emits or absorbs in 20,000 years
Humans Overwhelm Slow Geologic Changes
2. Climate Sensitivity High
- Antarctic ice forms if CO2 < ~450 ppm
- Ice sheet formation reversible
Humans Could Produce “A Different Planet”
Two conclusions should be emphasized. First the natural imbalance between geologic sources and sinks of CO2 is of the order of one ten-thousands of a ppm per year. In a million years that can cause a change of 100 ppm.
But the human-made rate of change is today about 2 ppm per year, about ten thousand times greater than the natural rate.
So the assertion that we should not be concerned about human-made climate change, because there have been much larger natural climate changes is nonsense. There have been larger changes, but on very long time scales. On any time scale of interest to humanity, humans will be in charge of the climate change.
The second conclusion is that we cannot burn all the fossil fuels, which would double or triple the amount of CO2 in the air, without setting the planet on a course to the ice free state. It would be a rocky trip, and it would take some time, as the ice sheets collapsed and sea level rose 250 feet. But it should not be doubted – feedbacks work in both directions – ice sheet formation is reversible.

27. Change in Sea Level During the Last Glacial and Interglacial Periods

28. Sea Level in North America if all Ice on Earth Melted

29. Extent of the Ice Sheet that Covered North America during the Last Ice Age

30. Temperature Variations During the Past 140,000 Years

31. Abrupt Climate Change: Our Worst Nightmare

32. Variations in Temperature During part of the Last Ice Age

33. The Younger Dryas and Other Abrupt Climate Changes

34. Several Abrupt Climate Changes