Climate up to Present Day

Introduction

This article will investigate changes in climate over

• 1,000
• 10,000 Holocene Period
• 100,000
• 400,000....years

According to the Intergovernment Panel on Climate Change there is an unequivocal warming of the climate. Some locations have increased their mean temperature by 2C in the last 150 years but overall the earth has warmed by 0.75C. The years 1996 to 2006 were the warmest in all of that time period. Two places cooled. Off the coast of Florida and the North Atlantic. The coolness of the Atlantic may be due to the slowing of the Gulf Stream Florida many have cooled in association with a shift in the phase of the North Atlantic Oscillation, which may spend more time in its positive phase under conditions of anthropogenic warming.

The last 1,000 years

Temperature readings over 150 years use Proxy Data based upon readings of Tree Rings, corals and ice cores. From this data and contempoary essays we know there was a Litte Ice Age in the 14th Century to 19th Century. There was a relative milder period called the Medieval Warm Period in 11th and 15th C with the warmest period of all being the 20th C

Not only are temperature recordings more accurate in the 20c but also climate forcings

The last 10,000 years the Holocene Period

The Holocene is generally considered to have begun around 11,700 years ago and lasted until the present day. Compared with climates experienced in earlier periods on Earth, it is one of relative warmth that represents the interglacial conditions that exist between Ice Ages, and there is no reason to assume that it signals the very end of the last Ice Age

Temperatures climbed steadily over approximately the next 6,000 years to a rather warmer period around the middle of the Holocene, at around 6,000 years before present. This relatively warm mid-Holocene period has been given many names, including the Holocene Climatic Optimum, the Hypsithermal and the Holocene Megathermal. Coming towards the present day, temperatures have cooled again somewhat relative to the Holocene Climatic Optimum. If we now focus in on much shorter timescale changes in just the last 1,000 years before present (BP), we can discern the Medieval Warm Period of around 1,000 years ago and the Little Ice Age that occurred around the year 1600 A.D. (approximately 400 years BP) Despite all these changes, when compared with climates of much longer ago, the Holocene period considered in its entirety is one of relative stability of temperature

To put these climatic changes into context relative to human existence on Earth, it is worth considering a few key dates in prehistory and the evolution of the modern human. Dinosaurs became extinct approximately 65 million years ago. Humans and apes evolved from a common ancestor, but the two diverged approximately 5–7 million years ago. Humans then split into two types around 2 million years ago. Genetic studies indicate that homo sapiens evolved as long ago as 250,000 to 400,000 years ago, while the earliest fossil records of anatomically modern humans are in Africa and date from approximately 200,000 years ago. Despite such early beginnings of humankind, it is generally believed that the Holocene period encompasses the entirety of civilised human life on Earth

The Last 10,000 years

On timescales of around 100,000 years, climate exhibits its largest known changes, varying into and out of Ice Ages. The main drivers of these variations are the so-called `Milankovitch cycles'these are variations in the Earth’s tilt and its orbital path around the Sun, which cause changes in the amount of energy it receives from the Sun. These changes are believed to provide the primary driving mechanism for shifts into and out of Ice Ages

The relatively warm periods between Ice Ages are known as interglacials. Earth is currently in an interglacial and the last one was approximately 125,000 years ago. When temperatures in the warm period between two glacial periods become colder, but not cold enough, or not for long enough, to be glacial, the period is termed a stadial. The Little Ice Age of around 1600 A.D. is an example of a stadial.

To put the climate of the recent past in the broadest context considered here. The figure shows the variation of temperature over the last 400,000 years as revealed by an ice core taken from Vostok in Antarctica. the ice contains other ‘information’ such as atmospheric fall-out of volcanic ash, wind-blown dust and pollen, and trapped air bubbles.

On this sort of timescale, we can now appreciate more easily that the climate of the last 10,000 years or so (the Holocene period) is about the most stable of any time in the last 400,000 years. According to the IPCC, sea level during the last interglacial was about 4–6 metres above that of the present day.

The nexxt figure reveals that present-day temperatures are not unprecedented; temperatures at the peak of all the previous interglacials were all warmer than at present, with some of them up to 3 °C warmer than the present day. What is unprecedented, however, is the atmospheric loading of the greenhouse gases carbon dioxide, methane and nitrous oxide. Figure 22 shows the variation of these gases over the last 600,000 years (with carbon dioxide depicted in red, methane in blue and nitrous oxide in green) and indicates that present-day concentrations of these gases (the starred values) are far in excess of any known over the last 600,000 years. The grey shading highlights the interglacial warm periods and indicates a marked correlation between high concentrations of greenhouse gases (particularly CO2 and CH4) and warm global temperatures. The science behind this correlation is very well understood. Thus we can be virtually certain that we are already committed to some degree of warming as a result of the additional atmospheric loading of greenhouse gases as a result of anthropogenic emissions, and that if we increase atmospheric concentrations of these gases still further we will commit ourselves to even more warming.