John Baez

October 1, 2006

Global warming is changing the world we know and love. But in fact, the Earth's climate has changed many times - especially in the last two million years, when a series of ice ages has wreaked havoc worldwide. We need to understand the past to have any chance of understanding the future... so here's a little tale of the Earth's climate. If you prefer charts to words, scroll down!

Though the Earth's temperature has been varying ever since its birth, I'll start my story at the beginning of the Cenozoic Era. The Cenozoic - the age of mammals - began with a bang around 65 million years ago when an asteroid smacked into Mexico and the dinosaurs died out. The Cenozoic is divided as follows:

Though the beginning of the Cenozoic - the Paleocene - was cooler than the hottest part of the Paleozoic, it was much warmer than today. Oxygen isotope ratios show the ocean was 10° to 15° Celsius warmer than today. For Americans who know nothing of metric units, that's 18°-27° Fahrenheit warmer than now. That's hot! 

55 million years ago, at the very end of the Paleocene there was an drastic incident called the Paleocene-Eocene Thermal Maximum. You can see it as the spike labelled "PETM" on this graph. Ocean surface temperatures worldwide shot up by 5-8°C for a few thousand years - but in the Arctic, it heated up even more, to a balmy 23°C (73°F). This caused a severe dieoff of little ocean critters called foraminifera, and a drastic change of the dominant mammal species. What caused this? Maybe a sudden release of greenhouse gases - carbon dioxide from volcanoes, or a "methane burp" released from gas hydrates on the sea floor. People have indeed found drastically different carbon isotope ratios at this time.

At the start of the Eocene, the continents were close to where they are now, but the average annual temperature in arctic Canada and Siberia was a balmy 18° C (65° F). The dominant plants up there were palm trees and cycads. Fossil monitor lizards (sort of like alligators) dating back to this era have been found in Svalbard, an island north of Greenland that's now covered with ice all year. Antarctica was home to cool temperate forests, including beech trees and ferns. In particular, our Earth had no permanent polar ice caps!

Life back then was very different than now. The biggest member of the order Carnivora, which now includes dogs, cats, bears, and the like, was merely the size of a housecat. The largest predatory mammals were creodonts, the size of wolves. But the biggest predator of all was not a mammal - it was Diatryma, the 8-foot tall "terror bird", with a fearsome beak! For more on this strange world and its end as the Earth cooled, see:

The temperature began to drop rapidly in the middle of the Eocene. By the Oligocene, 34-24 million years ago, glaciers started forming in Antarctica. The growth of ice sheets led to a dropping of the sea level. Tropical jungles gave ground to cooler woodlands.

What caused this? Some seek the answers in plate tectonics. The Oligocene is when India collided with Asia, throwing up the Himalayas and the vast Tibetan plateau. Some argue this led to a significant change in global weather patterns. But this is also the time when the supercontinent Gondwanaland finally broke up, with Australia and South America separating from Antarctica. Some argue that the formation of an ocean completely surrounding Antarctica led to the cooling weather patterns.

The beginning of the Miocene, 24 million years ago, is when grasses first became common. It's sort of amazing that something we take so much for granted - grass - can be so new! But grasslands, as opposed to thicker forests and jungles, are characteristic of cooler climates. And as Nigel Calder has suggested, grasslands were crucial to the development of humans! We grew up on the border between forests and grasslands. That has a lot to do with why we stand on our hind legs and have hands rather than paws. Later, the agricultural revolution relied heavily on grasses like wheat, rice, corn, sorghum, rye, and millet. As we ate more of these plants, we drastically transformed them by breeding. In return, they drastically transformed us: the ability to stockpile surplus grains ended our hunter-gatherer lifestyle and gave rise to cities, kingdoms, and slave labor.

So, you could say we coevolved with grasses!

Indeed, the sequence of developments leading to humans came shortly after the first grasses. Apes split off from monkeys 21 million years ago, in the Miocene. The genus Homo  split off from other apes like gorillas and chimpanzees 5 million years ago, near the beginning of the Pliocene. The fully bipedal Homo erectus dates back to 1.9 million years ago, near the end of the Pliocene.

Then, at the beginning of the Pleistocene, 1.8 million years ago, the Earth entered an even cooler phase, with jerky temperature variations causing a series of ice ages. Experts call them "glacials", and call the intervening periods "interglacials".

The latest glacial began around 110,000 years ago. As you probably know, people were around then. The Neanderthalers died out 35 thousand years ago, and the oldest cave paintings are 32 thousand years old. The glaciers reached their maximum extent around 21 thousand years ago, with ice sheets down to the Great Lakes and mouth of the Rhine, and covering the British Isles. Around 16 thousand years ago it started seriously warming up, and temperatures reached their present levels around 10 thousand years ago. We can see this from oxygen isotopes in ice from Greenland, and we can see it from the rise of sea levels.

In September 2006, scientists from NASA, Columbia University and elsewhere published a paper on climate change which shows, among other things, a graph of the Earth's temperature over the last million years or so. You can see a bunch of glacials on this chart. You can see that the temperature is now becoming hotter than since the end of the last glacial. With just one more degree Celsius of warming, it'll be hotter than ever in the last million years.

In other words, we may be witnessing the end of the whole cycle of ice ages!

As we get closer to the present the data gets more accurate and we can spot small fluctuations on top of larger trends - this is one reason it feels that things are happening faster. Though things have basically been warming up in the last 16 thousand years, there was a severe cold spell between 12.9 and 11.5 thousand years ago: the Younger Dryas event.

(I love that name! It comes from the tough little Arctic flower Dryas octopetala, whose plentiful pollen in certain ice samples gave evidence that this time period was chilly. Before the Younger Dryas there was a warm spell called the Allerød, and before that a cold period called the Older Dryas.)

Anyway, the Younger Dryas lasted about 1400 years. Temperatures dropped dramatically in Europe: about 7°C in only 20 years! In Greenland, it was 15° C colder during the Younger Dryas than today. In England, the average annual temperature was -5° C, so glaciers started forming. We can see evidence of this event from oxygen isotope records and many other things.

Why the sudden chill? One theory is that the melting of the ice sheet on North America lowered the salinity of North Atlantic waters. This in turn blocked a current called the "Atlantic thermohaline circulation", or the Conveyor Belt for short, which normally brings warm water up the coast of Europe. Proponents of this theory argue that this current is what makes London much warmer than, say, Winnipeg in Canada or Irkutsk in Russia. Turn it off and - wham!  - you'll get glaciers forming in England.

So, ironically, global warming may have brought on a sudden deep freeze in Europe. Some scientists are worried that we could be in for a repeat of the Younger Dryas if we keep melting the Arctic ice sheets at the rate we're doing now. For more, try:

On the other hand, some very respected scientists don't believe this theory of the Younger Dryas. For a fascinating discussion of the other view, try:

This talks about Lonnie Thompson, who studied ice cores from glaciers on tropical mountains. According to these cores, the impact of the Younger Dryas was even bigger far away from the North Atlantic. And, Thompson and others believe the importance of the North Atlantic as the Achilles heel of the climate system has been greatly exaggerated.

It's also questionable how important the Atlantic thermohaline circulation really is for the mild climate of Europe. It could just be the wind flow patterns! For a great discussion of this, try:

If a collapse of the Atlantic thermohaline circulation didn't cause the Younger Dryas, what did? I wish I knew.

Anyway, the Younger Dryas ended as suddenly at it began, with temperatures jumping 7° C. Since then, the Earth continued warming up until about 6 thousand years ago - the mid-Holocene thermal maximum. The earth was about 1° or 2° Celsius warmer than today. Since then, it's basically been cooling off - not counting various smaller variations.

But as we zoom in towards the present, we see more and more tiny details....

From 6000 to 2500 years ago things cooled down, with the coolest stretch occuring between 4000 and 2500 years ago: the Iron Age neoglaciation.

Then things warmed up for a while, and then they cooled down from 500 to 1000 AD. Yes, the so-called "Dark Ages" were also chilly!

After this came the Medieval climate optimum, a period from about 1000 to 1300 AD during which European temperatures reached their warmest levels for the last 4000 years.

From 1450 AD to 1890 there was a period of cooling, often called the Little Ice Age. This killed off the Icelandic colonies in Greenland, as described in this gripping book:

However, the term "Little Ice Age" exaggerates the importance of what's truly a tiny blip in the grand scheme of things. It was nowhere near as big as the Younger Dryas: temperatures may have dropped a measly 0.2° Celsius from the Medieval optimum, and it may have happened only in Europe - though this is a subject of debate.

Since then, things have been warming up. The subject has big political implications, and is thus subject to enormous controversy. But, I think it's safe to say this warming is much greater  than the other variations I've mentioned over the last 1000 years. The most accepted data shows Northern Hemisphere temperatures rising about 1.1° Celsius from 1900 to 2000.

More recently, in 2005, the National Climatic Data Center prepared a graph of the world's temperature from 1880 to 2005. It shows overall temperatures rising a bit more than 1° Celsius. It also shows that 1998 is the hottest year on record, followed by 2005. In 1998 there was an El Niño to blame for the heat; in 2005 there was not!

Lots of other studies have been done, and most of them roughly agree.

Of course, 1 Celsius ain't much compared to the 15°-20° Celsius cooling throughout the Cenozoic - but it's happening fast, and and it's not over yet! With all the changes the Earth has experienced over its history, one might think one more change is no big deal. In the long run, yes. But the future of humanity depends crucially on what happens in the "short run": the next millennium or two. If we didn't mess around with the climate, our Earth's climate might remain stable for another thousand years or more. As it is, we're bringing on more sudden changes.

Next, here's some of the data - charts of the Earth's climate, starting with the long-term history of the Earth, then zooming in towards the present.

Temperatures over the last 4.6 billion years

Here "Ma" means "million years ago". This chart was drawn from many sources; I got it from:

Deep ocean temperatures over the last 100 million years

These temperatures are estimated by the amount of oxygen-18; they're based on the 1981 work of Douglas and Woodruff, but I got this chart from:
Note the ancient past is to the right on this graph. One can see how the Earth has cooled throughout the Cenozoic, which began 65 million years ago.

Temperatures over the last 65 million years

This shows the Earth's temperature since the extinction of the dinosaurs about 65 million years ago - the end of the
Mesozoic and beginning of the Cenozoic. At first the Earth warmed up, reaching its warmest 50 million years ago: the "Eocene Optimum". The spike before that labelled "PETM" is a fascinating event called the Paleocene-Eocene Thermal Maximum. At the end of the Eocene the Earth cooled rapidly and the Antarctic acquired year-round ice. After a warming spell near the end of the Oligocene, further cooling and an increasingly jittery climate led ultimately to the current age of rapid glacial cycles - "ice ages".

This chart is taken from here:

Global ice mass over the last 6 million and 1 million years

The amount of ice is estimated by the amount of oxygen-18. In these charts up means "cold"! I got these charts from:

Conditions have become increasingly chilly and erratic in the Pleistocene - the last 1.8 million years. In the last 1 million years we see about ten Ice Ages, though it's hard to say what counts as an Ice Age.

Temperature over the last 5.5 million years

This figure is based on this paper:

Lisieki and Raymo combined measurements of oxygen isotopes in foraminifera from 57 globally distributed deep sea sediment cores. Beware: they constructed this record by first applying a computer aided process to align the data in each sediment core. Then the resulting stacked record was tuned to make the positions of peaks and valleys match the known Milankovitch cycles in the Earth's orbit. The temperature scale was chosen to match Vostok ice core data. So, there are a lot of theoretical assumptions built into this graph.

I got this chart and the explanation from:

Ocean temperatures over the last 2.5 million years

These temperatures are estimated by the amount of oxygen-18; I got this chart from:
The larger fluctuations are Ice Ages, mostly during the Pleistocene Epoch, which began 1.8 million years ago.

Temperatures over the last 1.35 million years

Note this timeline is not to scale! According to this study, after warming up 0.2° C per year for the last 3 decades, the Earth is now the hottest it's been in the last 12,000 years:
The current temperature matches the "Holocene maximum", a warm period about 12,000 years ago, right before the last ice age. If the temperature goes up another 1°C, it'll be the hottest it's been in the last 1.35 million years!

Temperatures over the last 1 million, 150 thousand, 16 thousand and 150 years

Here "ka" is an abbreviation for "thousands of years ago". These temperatures are estimated by various methods; I got this chart from: As we keep zooming in towards the present we keep seeing more detail:

Sea levels over the last 150,000 years

Higher sea levels correspond to warmer temperatures, since these make the ice caps melt. I got this chart from:
The dip in sea levels from 120 to 20 thousand years ago is due to the last Ice Age.

Greenland temperatures over the last 130,000 years

These temperatures are estimated by the amount of oxygen-18 in an ice core taken from Camp Century, Greenland. The curve has been smoothed to eliminate short-term fluctuations; it's based on the 1986 work of Dansgaard et al, but I got this chart from:
The colder period is the last Ice Age.

Greenland temperatures over the last 100,000 years

This chart is based on ice cores, taken from:

Greenland temperatures over the last 20,000 years

These temperatures are estimated by the amount of oxygen-18 in an ice core taken from Camp Century, Greenland. The curve has been smoothed to eliminate short-term fluctuations; it's based on the 1987 work of Boyle and Keigwin, but I got this chart from:
The cooling from approximately 11 to 10 thousand years ago is called the "Younger Dryas" event. (Other sources give the dates as about 13-11 thousand years ago.)

Greenland temperatures over the last 18,000 years

This chart is based on ice cores, taken from:

This is a good book for learning how people reconstruct the history of temperatures in Greenland from looking at a two-mile-long ice core drilled out of the glaciers there.

World temperatures over the last 10,000 years

I got this chart from:
After the Younger Dryas it warmed up for a while. The warm spell from 7 to 6 thousand years ago is called the "Holocene thermal maximum". After that it cooled again.

Temperature in the Northern Hemisphere over the last 1000 years

This is taken from
The warming since 1850 is evident here, as is the "Little Ice Age" around 1450.

World temperatures from 1880 to 2005

The temperature for 2005 only is based on 11 months of data; this will be fixed later. The first chart is from the Los Angeles Times:
It's based on the same data as the second chart, which comes from here:

Greenland temperatures over the last 100 years

This chart is based on direct observations, taken from:

Greenland: melting in 1992 versus in 2002

These charts are taken from:

Text © 2011 John Baez; charts copyrighted by those who created them