One of the problems with explaining temperature change over time, is understanding time itself.
Because inevitably, you run into this:
The Earth is around 4,500 million years old.
To put 4,500 million years into perspective, put your hands in front of you, about a metre apart, like this…
If you then equated that distance to a 100 year period, like this…
…then 4,500 million years would equate to around 40,000 km, which is roughly the circumference of the Earth.
If we applied a 100 year-to-90 cm scale to other significant events in the Earth’s and human history, it might look something like this…
|100||4 generations||90 cm|
|4.5 thousand||Pyramids at Giza||40 m|
|12.5 thousand||Start of Holocene||100 m|
|130 thousand||Modern humans||1.1 km|
|250 thousand||Early humans||2.2 km|
|500 thousand||Vostok ice core||4.5 km|
|2.6 million||Last ice age begins||23 km|
|50 million||Indian subcontinent collides with Asia||440 km|
|65 million||Last dinosaurs||575 km|
|200 million||Pangea starts to break apart||1,8oo km|
|400 million||First land plants||3,500 km|
|500 to 1,000 million||Geological records that indicate temperature change (approx)||4,500 to 9,000 km|
|3.9 billion||First life||34,500 km|
|4.5 billion||Age of Earth||40,000 km|
Yes, temperature has fluctuated over the Earth’s history.
Over 4,500 million years, the Earth has seen no ice at the poles, it has frozen over completely, and has seen everything between.
We know this because there is evidence—in the rocks, in the ice, and in the trees.
We call these proxies.
We can’t see evidence for the whole 4,500 million years. Yet, geological records allow us to understand temperature variations to around 500 million years ago. There is some evidence this extends to around 1,000 million years ago
Using the 100 year-to-90 cm scale, 500 million years would be about the same as the distance from London to Tehran. 1,000 million years would be London to Seoul.
This evidence tells us this time scale has experienced periods of extreme warming and cooling each lasting millions of years. It also reveals that the transitions between these periods lasted millions of years too.
When the Earth is hot, and the poles experience no ice and warm temperatures, we call it Greenhouse Earth.
The Earth has been in the state for most of its known existence.
The other extreme,Icehouse Earth, is much like the recent Ice Age, which began around 3 million years ago and continues today.
The following is simplified version of the broad fluctuations between Greenhouse and Icehouse Earth over the past 500 million years. Using the 100 year-to-90 cm scale this would equate to the distance between London and Tehran.
So what causes these million-year scale fluctuations?
They are driven by long-term, major events. These events either occur in isolation, or in conjunction with one another, to trigger the conditions that lead the Earth into a new climatic state. This includes:
The LAST 65 MILLION YEARS
Let’s take that 500 million year time frame and look at the last 65 million years.
The last 65 million years, or the distance between London and Western Germany, provides further context for the recent warming.
Apologies in advance! the x-axis now flips from left-to-right to right-to-left.
This graph demonstrates a gradual decline in temperature over the last 65 million years. It measures temperature by using oxygen isotope measurements in foraminifera fossils as proxies.
(1) The Paleocene–Eocene Thermal Maximum (PETM) — a sudden spike in temperature driven by a massive carbon release into the atmosphere.
The PETM caused a temperature increase over 20,000 years of another 5°C on top of that — leaving little to no ice on Earth.
While the exact cause of the carbon release is contested, the event has allowed scientists to understand a very unique climatic event. The event has shaped scientists’ understanding of today’s climate change
Estimates of carbon release during the PETM range between 3,000 and 7,000 gigatonnes over a 3,000 to 20,000 year timeframe.
On a carbon per year average, it would look something like this, with the low end estimate on the left, and the high end estimate on the right:
To give you an idea of today’s rate of emissions, this is how these two PETM estimates are dwarfed alongside emissions averaged over the last 250 years…
Back to this…
(2) The Earth then enters a period of long term cooling, in part due to the collision of India and Eurasia, which causes the Himalayas to rise.
The cooling continues, with a major cooling event occurring around 30 million years ago and ice sheets returning to polar regions.
The cooling event occurs over thousands of years.
In fact, the changes mentioned above, sometimes gradual, sometimes abrupt, each occur over several thousands or millions of years.
Using the 100 year-to-90 cm scale, this represents kilometres of distance.
(3) Around 25 million years ago, the Earth warms again, before gradually cooling. Earth’s ice sheets begin to form their present day size and thickness.
(4) Alongside these broad fluctuations, shorter fluctuations are observed.
The ‘shorter’ fluctuations
The Earth has fluctuated between periods of extreme warming and cooling that have lasted of millions of years.
We are currently within one of those cooling periods—Icehouse Earth.
Within icehouse periods, the Earth experiences glacial and interglacial periods, which last thousands of years.
|Glacial period||Interglacial period|
*Polar and mountain ice sheets aplenty
*Ice sheets blanket North America and northern Europe
*Pretty much like now
Here are the last 800 thousand years. The fluctuations in carbon dioxide (CO2) are driven by shifts between glacial and interglacial periods.
800,000 years equates to 7 kilometres down the road.
But what’s causing the fluctuations between glacials and interglacials?
In 1938, Serbian scientist Milutin Milankovitch observed that the most likely cause was the variation in the intensity and timing of heat from the sun.
He pointed out three such variations: procession, tilt and eccentricity.
- Procession of the equinoxes: The wobble of the axis, every 22,000 years, or 200 metres
- Tilt: The obliquity of the Earth’s tilt, every 41,000 years, or 350 metres
- Eccentricity: The switch between circular and elliptical orbits, every 100,000 years or 900 metres
And over thousands of years, these orbital processes influenced a series of other changes in the climate system. This includes melting ice sheets, which alter ocean circulation and transfer of heat around the Earth.
These processes ultimately change the climate.
The regularity of the orbital changes means the Earth switches between glacial and interglacial cycles periodically.
We are currently in an interglacial period, the green spike circled in red.
This period is otherwise known as the Holocene.
Ahh the Holocene —comfortable, stable climate conditions that have allowed human civilisation to flourish.
The Earth first entered the Holocene around 12,000 years ago; that is, about 100 metres.
From changes over millions of years and fluctuations over thousands of years, we are left with just over ten thousand years—the last little green chunk in the graph above.
And it is at this point, that everything else falls into place.
The last 12,000
A re-constructed record of global temperature over the last 12,000 years shows a period of stability, with a gradual decrease followed by a very sharp increase.
Over last two thousand years, about 20 metres, the trend becomes more clear.
The year 1850 is a big deal.
We finally have instruments that can measure temperatures, and we have a long enough period to assess change.
Over this period, we can see a clear increase in global temperature, with a sharp increase since the 1970s.
And so here we are, a short history of temperature change throughout time.
Yes, Earth’s climate has always changed.
Sometimes it’s been covered in ice, sometimes reptiles have lived in Arctic regions.
Denialists will point to this as evidence that the fluctuations are part of natural cycles, casting doubt on today’s human-induced change.
However, the rate of change in the past compared to recent history means this time it’s different.