User:Sirosser/Historical Climate Change

Historical Climate Change
When talking about historical climate change, Earth's climate can be looked at in terms of it's recent history, where we mean very recent on a geological scale, i.e. in the last hundred years or so, which is a minuscule period in Earth’s history, bearing in mind it’s some 4.5 billion years old, or significantly longer periods, in the order of hundreds of thousands of years. One method of assessing what the climate may have been like at a specific period of Earths history is to look at the levels of Carbon Dioxide (CO2) in the atmosphere at a particular period in time, as higher levels of CO2 which is a greenhouse gas, tend to point to a warmer atmosphere. Indeed, accurate measurements of CO2 have been recorded by Charles David Keeling since 1958, from the Mauna Loa observatory in Hawaii, and these show that CO2 levels have risen from 315 parts per million (ppm) to 385 ppm during the last fifty years. It also seems that an increase in CO2 levels is linked with an increase in temperature..

Just a natural variation?

Science has enabled us to look into Earth’s distant climatic past, which may just give us an insight as to what we can expect in the future. The study of past climate, or palaeoclimatology to give it its scientific name, reveals that a huge variation in climate has taken place over millions of years of Earth history. When we think about the distant past, of the days when dinosaurs were roaming the Earth, the vision that is usually conjured up is of a hot and balmy climate. Indeed there is evidence that the Earth’s oceans 65,000,000 years ago were about 10 to 15°C (18 to 27°F) warmer than today. We also know that the Earth has, in the past, been much colder, cold enough to plunge it into the grip of many an Ice Age.

During the last fifty years or so, scientists have made huge advances in unlocking Earth’s remote climate by looking at data and evidence preserved in tree rings, sea corals, and pollens (biological data). Then there are rock and other sediments from sea and lakebeds (geological data), and the most revealing is frozen ice-core data from the Antarctic and Greenland ice sheets (cryological data).

We know that the Earth is about 4.5 billion years old by using a process called radiometric dating. Mass spectrometry is used to date the element uranium, which decays at an incredibly slow but predictable rate. Carbon is also used to age-date material (carbon dating), but can only be used to date relatively young material – about 50,000 years old. These methods allow scientists to tell roughly how old something is, and this technique is used to date a variety of data/materials mentioned above.

Chilling data

Ice cores have proved to be invaluable in revealing what the Earth’s climate has been like in the past. Scientists have managed to extract cylindrical cores of ice, typically about four inches in diameter, and extending to over three kilometres deep into the Antarctic and Greenland ice sheets.

How is this done? As snow forms, it crystallises around minute particles in the atmosphere. Microscopic bubbles of gas get trapped as layer upon layer of snow falls to the ground, which can then be analysed to reveal what the temperature, CO2 and methane levels were at any given time in the past.

The Vostok ice cores taken from Antarctica records climatic data going back about 400,000 years, but the deepest ice cores, recently extracted from the Antarctic by a European project (EPICA), have pulled out the deepest core allowing a glimpse back some 900,000 years in time.

What do ice core records reveal about Earth’s climate?

If we take a journey back through Earth’s climatic history, we find that between about 1350 and 1850 there were two distinct cold periods, referred to as the Little Ice Age, when temperatures were about 1°C (1.8°F) lower than today’s.

Rivers and lakes inthe northern hemisphere would regularly freeze over during winter, and indeed the Thames in London, over a period of about 400 years, would freeze over on average every twenty-five years or so. The freezing of the river may have been assisted, however, by the slower-moving water due to old London Bridge in situ at the time. If we go back further, from about 1000 AD to about 1300, we arrive at a period known as the Medieval Warm Period. Temperatures during this time were warmer than they had been for thousands of years before that, but similar to now.

Further back still, from between 10,000 and 15,000 years ago, we arrive at the end of the last Ice Age or glacial period. The period from then, which we are still in, is called the Holocene Interglacial Period, and marks a time when global temperatures started to increase as the grip of the last Ice Age ended. We are reminded that we are indeed in an interglacial period, which suggests at some point in the future there is likely to be another ice age, unless of course global warming either prevents or delays this from happening. The last Ice Age lasted for about 100,000 years, and during this period massive ice sheets up to 2 kilometres deep (1.24 miles) extended from the North Pole as far down as New York. Sea levels as a result of all the ice fell by about 100 metres (328 feet) below current levels.

Antarctic ice-core data reveals that during this period, about 20,000 years ago, temperatures were almost 8–10°C (about 14–18°F) lower than present.

As we go back further still, to about 115,000 years ago, the records show that temperatures started to rise again. Indeed this indicates that there was another interglacial period from about 115,000 to 140,000 years ago. There was a corresponding increase in temperatures by almost 8–10°C (14–18°F) during this period, bringing temperatures up to a level not much different from what they are today.

Further back again, we enter another ice age. From about 140,000 years ago to about 240,000 years ago the Earth was in the grip of another 100,000-year-long ice age, with a corresponding drop in temperature by almost 8–10°C (14–18°F) for most of that period.

The ice records reveal that this cycle has repeated itself at least eight times, and sediment cores show evidence of ice ages occurring on a cyclical basis over the last million years, every 10,000 to 15,000 years or so, and lasting for about 100,000 years each time. There were ice ages lasting a relatively shorter 40,000 years or so for millions of years prior to that. The timings of the ice ages may be linked to the Earth’s orbit around the sun. So, we know that the Earth, during the last 650,000 years at least, has been gripped by several 100,000-year-long ice ages, with 10,000 to 15,000-year-long interglacial warm periods, when temperatures fluctuated by about 10°C or thereabouts (18°F). While the Antarctic ice-core records show temperature variations of +/ - 10°C (18°F) or thereabouts between glacial/interglacial periods, mean global temperatures would probably have varied by about fifty per cent of that of the poles, i.e. by about 5 to 6°C (9 to 10.8°F). It is clear therefore that Earth’s climate has cycled from ice age to interglacial over the millennia, and during our current interglacial period there has been some small, one to two degree plus or minus variation in temperature that has resulted in the Medieval Warm Period and the Little Ice Age. In fact the Earth has experienced a series of ice ages during the last 2.6 million years or so.

CO2 levels and ice-core records

Scientists have discovered that CO2 levels are now higher than at any time in the past 650,000 years. It is also the case that when the data relating to CO2 is compared with past temperatures, it can be seen that with CO2 levels at their highest, so are temperatures, and vice versa, and these measures in turn correspond to Earth’s past glacial and interglacial periods.

The lower the level of CO2, the lower the temperature of Earth is in a glacial period. As CO2 levels increase, so does the temperature, which corresponds to interglacial periods in Earth’s history. CO2 levels are now about 385 ppmv in air, and increasing compared to a 650,000-year range of between 180 and 300 ppmv, as revealed by the ice-core records.

Does CO2 lead temperature or lag behind it?

While higher levels of CO2 go hand-in-hand with higher temperatures, does this mean that higher CO2 levels have caused the rise in temperature, or could it be the other way round? Some scientists dispute the theory that higher CO2 levels are causing higher temperatures, as there appears to be approximately an 800- year time lag between CO2 rises and temperature increase. While the ice-core data cannot be questioned, it seems the data may be open to interpretation, meaning that higher temperatures cause higher levels of CO2 rather than the other way round. Ice-core records show CO2 changes over very long timescales, that is glacial to interglacial time periods, and while CO2 levels and temperatures do appear to be inextricably linked, the science community now think that temperatures may be the first to rise during interglacial periods (less ice equals less reflection of sunlight, which equals warmer temperatures). Warmer temperatures mean higher greenhouse gas concentrations. CO2 and ice volume should therefore lag behind temperature somewhat when looking at glacial to interglacial timescales.

Global average temperature was lower during glacial periods for two main reasons: 1 CO2 levels were only about 190 ppm in the atmosphere, and other greenhouse gases were lower. 2 The Earth’s surface was a lot more reflective because of the presence of much more sea and surface ice, giving a much greater albedo. It is thought that the second factor has the greater influence, creating two-thirds of the total radiative forcing, with CO2 and other greenhouse gases the other third. Therefore while temperature is certain to rise based on the current levels of CO2 (385 ppmv), the second factor (above) needs to be taken into account when extrapolating temperature against ice-core CO2 records. In other words, CO2 and other greenhouse gases may be responsible for only just over thirty per cent of the radiative forcing or warming found in the ice-core records.

Historical proxy data reveals an event that occurred in Earth’s distant past, about 55,000,000 years ago. The event caused global mass extinction, some 10,000,000 years after the killer asteroid that ended the dinosaurs’ reign on Earth.

Palaeocene-Eocene Thermal Maximum

The Palaeocene-Eocene Thermal Maximum (PEMT), as it is known, saw a sudden global spike in air and sea temperatures over a period of only a few thousand years. It is thought that ocean temperatures went up by about 5 to 8°C (9 to 14.4°F) during this period. Two scientists from the Scripps Research Institute in California found, from looking at deep-sea sediment cores, that an underwater conveyer-belt-like process, in which cold and salty

water exchanges with warmer surface water, virtually shut down in the southern hemisphere, and started up in the northern hemisphere. This apparently drove warmer water to the deeper sea, possibly releasing the previously frozen methane gas hydrates discussed in the last chapter. This would have caused a sudden massive spike in greenhouse gases, which would have warmed the Earth further, resulting in a mass extinction of bottom-dwelling marine life and mass migrations on land as animals adjusted to the new climate.

Younger Dryas period

A more worrying and slightly more recent rapid climate-change event occurred about 14,500 years ago, towards the end of the last Ice Age, called the Younger Dryas period. The period is named after an Arctic-alpine plant, Dryas, which populated Europe during these cold conditions. The Earth’s climate warmed fairly rapidly about 14,500 years ago, only to change suddenly again to conditions more akin to the Ice Age that had just ended. This glacial spell lasted for about 1,000 years. Then, about 11,500 years ago, temperatures rose by about 10°C (18°F) in a decade or so, incredibly quickly. This sudden change in climate from relatively warmer to ice-age conditions is thought to have been triggered by the breakdown of the ocean water circulation system, which brings warmer water and air to the northern hemisphere from the tropics. This, in turn, is thought to have been caused by a meltwater surge from Antarctica or North America, as ice sheets began to melt, resulting in huge amounts of freshwater flowing into the ocean. This has been termed Meltwater pulse 1A following a ‘…defined sea level rise of about 16-24 metres (52-79 feet) about this time. A further surge of meltwater took place after the Younger Dryas period known as Meltwater Pulse 1B, when meltwater from glacial Lake Agassiz, southwest of Hudson Bay in Canada drained into the North Atlantic.’

The ocean thermohaline circulation still plays an incredibly important role in maintaining northern-hemisphere temperatures. If these rapid climate-changing events can occur in the Earth’s distant past, what’s stopping something similar from happening again if the Earth’s oceans warm up and glaciers and ice shelves start to melt as a result of increased greenhouse-gas warming? A sliding-scale graph of ice-core data going back in time 400,000 years, from the Vostok Antarctica ice core, can be seen on NASA’s Earth observatory website.