4.5 Billion Years of the Earth’s Temperature

This blog post will give a brief overview of how the Earth’s temperature has changed up to the present day. We only have actual temperature measurements going back a couple of hundred years however there are several other methods we can use to give reliable estimates of the Earth’s temperature in the past. ‘Proxy’ measurements include rock sediment sampling, tree rings and ice cores. The video below gives a good introduction to how the British Antarctic Survey use ice cores to generate accurate atmospheric gas and temperature records going back 800,000 years! The diagram below shows an overview of the Earth’s temperature from 500 million years ago to the present and may help with picturing the changes in temperature when reading this post.

 Very Early Earth’s History (4.5 billion – 3.8 billion years ago)

The Earth was formed roughly 4.5 billion years ago. Until 3.8 billion years ago it was a completely inhospitable environment with the surface being mainly molten lava. The Earth eventually cooled enough for its crust to form. Land masses could then exist and, when it was cold enough to rain, the oceans formed.  Around this time the atmosphere was predominantly consisted of methane (CH₄) and ammonia (NH₃), two extremely important greenhouse gases, thus their radiative forcing kept the Earth’s atmosphere warm and toasty!

The Oxygen Explosion (2.5 billion – 500 million years ago)

Oxygen (O₂) in the atmosphere was almost non-existent until ~2.5 billion years ago. The evolution of cyanobacteria, which produced oxygen as a bi-product of photosynthesis, meant that O₂ levels dramatically increased. This rapid change in atmospheric composition caused widespread extinctions of most of the previous anaerobic bacteria. This ‘new’ atmosphere made the Earth much colder as there were no longer bacteria emitting radiative forcing-methane and carbon dioxide into the atmosphere. It is thought that the average temperature at the equator was roughly the same as current Antarctic conditions!

History of the Earth’s Temperature. originally sourced from here.

500 – 250 million years ago

During this period the Earth’s atmosphere became more stable, eventually cooling to similar temperatures to today’s average (see first section on plot above where the temp change is ~0 ΔT).

Animal Evolution (250 – 65 million years ago)

During this time the evolution of aerobically respiring animals occurred, i.e. DINOSAURS! This meant the concentration of CO₂ increased and global temperatures increased again. We know that there was a sudden decrease in temperatures around 65 million years ago which resulted in the extinction of the dinosaurs. The most widely accepted reason for this is a massive comet hitting the Earth sending huge amounts of matter (read: aerosols) into the atmosphere. This caused a global decrease in temperature due to an increased albedo effect (for more information about this and the contribution of aerosols to this effect please read my previous blog: An introduction to aerosols).

Thermal Maximum (55 million years ago)~55 million years ago, records show a massive warming of between 5-8 ⁰C in just 20,000 years (It is thought that during this time it was so warm palm trees could have grown in the poles!). The direct cause is still disputed amongst scientists, however it is generally agreed that a sudden release of carbon into the atmosphere caused the warming. This was probably in the form of methane from either the ocean bed or from within ice structures called clathrates. It was after this period that mammals started to evolve.

 

Ice Age (35 million years ago)

The thermal maximum continued to around 35 million years ago when the Earth cooled into the Ice Age. The theory behind this change in temperature is that a type of fern named Azolla became extinct. The Azolla then sank to the bottom of the ocean, taking with it much of the carbon absorbed as carbon dioxide, therefore removing it from the atmosphere. With the carbon dioxide not present to act as a greenhouse gas, global temperatures decreased again. Unlike the last period of cooling, this time the Earth had fully formed continents, including mountain ranges, and land mass at the South Pole (Antarctica). This new land coverage helped amplify the cooling via circulation.

An ice age is defined as when a planet’s poles are covered with ice, so technically we are still in one! Within an ice age there are periods of glacials and inter-glacials. Glacials are episodes of colder temperatures whereas inter-glacials are warmer time phases. Both will last several thousands of years. These changes in climate can be explained with the Milankovitch cycles (please read post #2 – The Milankovitch Cycles – for more information). NB: You can see on the plot above sections labelled ‘the mini ice age’ and ‘the medieval warming’ period. I plan to do future blogs on these events as this post is getting far too long!

Recent Warming (1880 – present day)

The warming we have seen in recent years has been like nothing experienced before in the Earth’s history. The last 100 years of warming has cancelled out the previous 6000 years of cooling that occurred before. The video below (sourced from NASA) shows just how dramatic the rate of global warming is over this time period.

Thanks for reading to the end of this post, it ended up a bit too long! Next time I want to introduce an event called the Northern Atlantic Oscillation: the phenomenon that is thought to have caused the medieval warming period.

A brief Introduction to Global Warming

Hello and welcome to my first blog post! I can’t really start without an explanation of how global warming occurs, as this is one of the most fundamental reasons why climate science exists! So here we go…

The Earth’s atmosphere allows life as we know it to exist. Without it, global temperatures would be around 33-35 ^oC colder. The gases that make up our atmosphere can absorb some of the sun’s energy leading to an increase in temperature. Energy from the Sun reaches the Earth in the form of light and heat. The Earth has the ability to absorb all of this radiation but some is reflected back into space either by the Earth’s surface or by its atmosphere. This process is called the Albedo Effect and without it the Earth would be much warmer. The Earth re-radiates energy at a slightly longer wavelength that can be absorbed by some atmospheric gases. These gases then re-radiate this as thermal energy.

A measure of how strong an effect this re-radiated energy has on the Earth’s temperature is called a gas’ radiative forcing. Radiative forcing is the difference between radiant energy received by the earth and energy radiated back into space, and its units are in watts per meter squared (Wm^-2). The gases that contribute most to the greenhouse effect, in terms of their radiative forcing, their lifetimes and also their abundance in the atmosphere include carbon dioxide (CO₂), water vapour (H₂O) and methane (Ch₄). The relationship between temperature and concentrations of these gases is strikingly positively correlated, as the graph below shows. In another blog I plan to explain more about how the Earth’s temperature has changed throughout geological history.

Record of the Earth’s temperature and carbon dioxide concentrations taken from ice core data in Antarctica.

Almost all atmospheric gases have both natural and anthropogenic (man-made) sources. In fact, for most of the major greenhouse gases natural sources outweigh man-made. Since the Industrial Revolution, however, anthropogenic emissions have been rapidly increasing. The plots below, published by the Intergovernmental Panel on Climate Change (IPCC), show just how high the atmospheric concentrations have risen. It is this increase, or offset from the pre-industrial balance between sources and sinks, that is causing temperatures to rise. For more information on the IPCC and its soon to be released 5th assessment report click here.  Or if you want a more detailed summary of all the aspects of climate change this ‘summary for policy makers’ is really interesting.

Time series of Methane (CH4), Carbon Dioxide (CO2) and Nitrous Oxide (N2O) emissions for the last 10,000 years.

Of course natural variability also affects the earth’s climate and can contribute to global warming. Examples of natural variability include variations in the Earth’s orbits (i.e. Milankovitch cycles – more to come in another blog), solar radiation variability, lunar tides and interactions between the oceans and the atmosphere. The world’s scientific community agree that the Earth is currently warming however there is a degree of discussion about how much human impact is contributing to global warming. The IPCC has stated that ‘the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations’. Very likely being classed as a >90% probability.

Thanks for reading what I hope is the first of many blog posts.  Next up I will discuss changes to climate that occur on longer times scales, i.e changes to the Earth’s orbit about that sun.