The Earth has undergone many episodes of natural global warming and cooling and by various causes. The Earth's most common mechanism for climate change are Milankovitch cycles—variations in the Earth's orbit that change its distance from the Sun, which spur ice ages and subsequent warming. Other changes in Earth's past climate were caused by the same processes causing today's warming. The Paleocene-Eocene Thermal Maximum (PETM), which occurred around 56 million years ago, is the most recent event that we can compare today's warming to. Global temperatures rose at least 5°C (9°F), and the PETM warmth lasted 200,000 years before the Earth system was able to remove the extra CO2 from the atmosphere. The resulting impact on Earth's climate was so severe that a new geological era was born—the Eocene. Earth's ecosystems were able to adapt to the PETM because the warming was gradual; however, the warming we're causing today is about 10 times as fast, and Mother Nature might not be able to keep up with the changing climate this time around.
Global Warming 56 Million Years Ago
After years of research, the PETM is now thought to have been caused by greenhouse gas emissions, similar to how the earth is warming today. 56 million years ago, at the end of the Paleocene epoch, the supercontinent Pangaea was in the final stages of breaking apart into the continents as we know them today. As the land masses split apart, volcanoes erupted and molten rock bubbled toward the Earth's surface, literally baking carbon-rich sediments and releasing the greenhouse gas into the air. During this time, atmospheric temperature probably increased by a couple of degrees.
The initial increase in temperature triggered events that led to more greenhouse gas emissions and more warming. Climate scientists generally agree that the feedback with the most impact on the atmospheric temperature increase was the melting of methane hydrates in the ocean seabed. As the atmosphere warmed the ocean surface, currents (probably not unlike the thermohaline circulation we know today) would have funneled the warm water to the ocean floor, where it melted the frozen methane hydrates (also referred to as methane clathrates), releasing the potent greenhouse gas into the ocean and eventually the atmosphere, a process called outgassing. Hydrates could have also been outgassed via other mechanisms—tectonic uplift, volcanic activity, or changes in deep ocean temperature from the closing off of certain gateways due to shifting continental plates. No matter how the process started, methane (CH4) is a potent greenhouse gas that has 20 to 25 times more warming power than carbon dioxide, although it degrades to carbon dioxide after about ten years in the atmosphere. However, a steady influx of the gas would have been sufficient to warm the planet by more than a few additional degrees.
Figure 2. Rate of temperature change today (red) and in the PETM (blue). Temperature rose steadily in the PETM due to the slow release of greenhouse gas (around 2 billion tons per year). Today, fossil fuel burning is leading to 30 billion tons of carbon released into the atmosphere every year, driving temperature up at an incredible rate.
Many of the other climate feedbacks that we either already observe today or expect to experience probably took place during the PETM warming, as well. Severe drought would have led to increased wildfires, injecting more carbon into the atmosphere. Some research shows that permafrost on a then glacier-free Antarctica thawed, which would have also released carbon dioxide and methane. Another interesting source of carbon that some scientists hypothesize is the burning of peat and coal seams. Peat is decayed vegetation and has a very high carbon content. Peat, which is found in the soil beneath the surface, can be ignited by something like a wildfire and continue to smolder for as long as centuries. Coal seams can be ignited in a similar way, and burn for decades to centuries, releasing huge amounts of carbon into the atmosphere.
PETM Warming vs. Current Warming
During the PETM, around 5 billion tons of CO2 was released into the atmosphere per year. The Earth warmed around 6°C (11°F) over 20,000 years, although some estimates are that the warming was more like 9°C (16°F). Using the low end of that estimated range, the globe warmed around 0.025°C every 100 years. Today, the globe is warming at least ten times as fast, anywhere from 1 to 4°C every 100 years. In 2010, our fossil fuel burning released 35 billion tons of CO2 into the atmosphere. By comparison, volcanoes release 0.2 billion tons of CO2 per year. How fast carbon enters the atmosphere translates to the how fast temperature increases, and the environmental and societal consequences of warming at such a break-neck speed could be devastating.
|Continental drift, volcanoes, methane hydrate melting, fires, permafrost melting||Anthropogenic burning of fossil fuels (oil, coal, natural gas, etc)|
|Around 5 billion tons per year||At least 30 billion tons per year|
Rate of warming
|0.025°C per 100 years||1 to 4°C per 100 years|
|Ocean circulation reversed, oceans acidified, permafrost melted, peatlands and forests burned in wildfires||Observed impacts: significant sea ice decline, extreme drought, more wildfires, increase in glacier melt, more catastrophic floods, ocean acidification, sea level rise, shoreline erosion Potential impacts: degraded air and water quality, permafrost melting, global ocean circulation changes, more violent winter storms and spring tornado seasons, more intense hurricanes|
Ecosystem & human impact
|Migration of land mammals, extinction of some benthic foraminifera, coral bleaching||Observed impacts: Famine and malnutrition due to drought, coral bleaching, species endangerment (e.g. polar bears, marine turtles, North Atlantic whales, giant pandas, orangutans, elephants) Potential impacts: increased mortality from extreme weather and malnutrition, increase in disease vectors, decrease in agricultural yield, mass wildlife migration and extinction, total societal collapse|
Environmental impacts of the PETM were similar to the impacts that are warned of today. There is some evidence that during the PETM, large-scale ocean circulation reversed, which would have led to enhanced warming. Ocean circulations are largely by temperature and salinity (salt concentration), and warming of ocean water at high latitudes would have acted to at least slow, if not totally reverse, the "global conveyor belt."
Sea Level Rise
Since the PETM occurred in an already warm climate (another thing that sets the PETM apart from modern warming), there was very little sea ice and glacial cover to melt, so sea level did not change dramatically. However, there is plenty of ice to melt on our modern planet, and we expect sea level to rise anywhere from 0.2 to 0.6 meters (0.7 to 2 feet) by the year 2100. We've already seen sea level rise at a rate that would support the higher end of that range (0.6 meters/2 feet).
Permafrost and Methane Hydrates
Scientists agree that a major contributor to the PETM warming was the melting of methane hydrates on the seafloor and permafrost at high latitudes. Both of these store immense amounts of carbon and constitute a tipping point for the climate—once hydrates and permafrost begin to melt, the process will be irreversible. The reservoirs of methane hydrate stored in marine sediments (500 to 10,000 billion tons of carbon) and in permafrost (7.5 to 400 billion tons) are being constantly monitored. Melting of methane hydrates and permafrost enhanced the PETM warming, and could tip the scales of modern warming, too. Already we see that permafrost is degrading, and scientists suspect that methane hydrates are melting near the Arctic Shelf.
The most disruptive impact during the PETM was likely the exceptional ocean acidification. The ocean naturally absorbs carbon dioxide from the atmosphere, and also from the sea floor (in the form of calcium carbonate). When excess carbon enters the atmosphere, the oceans try to balance the system by absorbing more. Numerous studies have shown that this was the case during the PETM. The effect of this is a decrease in the pH of the water, or "acidification." Unfortunately, this has a devastatingly negative impact on calcifying creatures like foraminifera, molluscs, and coral. Coral bleaching is caused by a number of environmental changes, including ocean acidification and increased water temperature.
Ecosystems adapted remarkably well to the PETM warming, likely because it was gradual enough for life to adjust to the new environment. The only species extinction that scientists have found were some foraminifera that lived on the sea floor. It's hypothesized that these foraminifera could not adapt to the new warmth at such great depths (bottom waters warmed 4 to 5°C). As excess carbon dissolved in the ocean, the water acidified which likely resulted in coral bleaching. Marine life adapted by migrating poleward toward cooler water. On land, mammals not only migrated to find more sustainable environments, but they also decreased in size, likely because it is easier for smaller animals to dissipate heat. Hoofed animals and turtles were confined to the tropics before the PETM, but during the warming this animals made the trek northward into North America and Europe. The PETM did not cause mass extinctions of plants and animals on land, but a major turnover in mammalian life occurred at that time. Many of today's major mammalian orders emerged in the wake of the PETM.
Modern ecosystems are already struggling to adapt to their new, warmer environments. Penguins, polar bears, whales, seals, salmon, and orangutans are just a few of the mammals being impacted by anthropogenic climate change. Foraminifera have already decreased markedly in some areas. Coral is bleaching at a very rapid rate. While it was possible for land mammals to migrate to cooler regions in the PETM, manmade infrastructure (roads, railways, cities, etc) will prevent them from doing so this time around. Given the rate of warming the globe is experiencing, it is likely that many ecosystems will be totally incapable of adapting.
There are a lot of uncertainties surrounding the PETM—this extremely warm geologic period has been notoriously difficult to recreate, but recent advancements in understanding the warming have been made. Uncertainties should not be interpreted as misunderstanding. Instead, they should be treated a testament to how sensitive the climate system could be, and how influential humans are on the delicate global energy balance. It is clear that the earth dumped almost all of its stored carbon into the atmosphere, and now we are doing the same by pulling fossil fuels out of the ground and burning them. Just like the previous great global warming did, we are likely catapulting ourselves into a new geologic era: the Anthropocene.
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