When the temperature changes so much every day, why does 1ºC or 2ºC matter globally?

October 2023

Why do scientists make such a fuss about a 1ºC or 2ºC increase in average global temperatures? Even at the equator the temperature changes much more than that each day. In some places the difference between night and day can be 30ºC or more. Why all the alarm bells?

To understand why such a small change matters we need to look at global temperatures long term.

We’ll look first at the last million years, and then the last 12,000 years, before going back further, to the last 65 million years.

The last million years is shown in the chart below. It shows the average temperature in degrees centigrade. The source is here. As you can see, the average temperature over the last million years on Earth was 12.5ºC.

On the right, at the bottom, you can see the end of the last ice age. Then, the temperature was more than 3ºC lower than the long term average. Over the following 10,000 years, the ice sheets melted and the temperature gradually rose from just over 9ºC, far below the average, to 14.5ºC, far above the average.

Chart 1: The average global temperature over the last million years

This rise in temperature over thousands of years followed a long pattern, which you can see. Over the last million years, there have been long periods of warming which have lasted thousands of years. These have been followed by thousands of years of cooling. The pattern is the result of small changes in the earth’s orbit.

The last 2,000 years is shown on the right, at the top. Given the scale, these 2,000 years are about 1mm long. What’s important to note is that the temperature at this time was very high compared to the last million years. The last 2,000 years have been 2ºC warmer than the long term average.

Note: this chart does NOT show what has happened in the last 200 years.

Now let’s look at the last 12,000 years.

On the chart below, the horizontal axis shows the last 12,000 years, from the end of the last ice age until today. The vertical axis shows the temperature anomaly. It shows how the temperature compares to the average during the 20^th century. The various lines on the chart are different academic estimates for the temperature. The thick black line is the average of these. The chart comes from here. I have simplified it slightly and updated it.

Chart 2: The average global temperature over the last 12,000 years

The last centimetre on the right of the chart shows the last 1,000 years. There are two notable fluctuations. There was a time around 1,000 years ago when the temperature rose and it became slightly warmer. This was the Medieval Warming Period. It is when grapes grew in Scandinavia. This period did not change the global average very much, because the warming mostly affected the northern hemisphere.

There is also a period of several hundred years when the average temperature fell. This is known as the Little Ice Age (LIA) and is shown on the right of the chart.

Climate model simulations have identified multiple causes for this cooling, with volcanic activity playing an important role. Even so, the Medieval Warming Period and the LIA are used by climate deniers to suggest that what’s happening today is part of a natural cycle. But what’s happening today is far outside the range of any historical cycle, in both scale and speed.

During the LIA, it took 400 years for the average temperature to drop by around 0.6ºC. It took 200 years to recover.

What’s been happening since 1800 is different. The average temperature has changed much more, and much faster than before. As you can see on the right of the chart, it has risen from 0.4ºC below the long term average to far above the average. In September 2023 it was 1.8ºC above the average. Compared to the LIA, the temperature in the last 200 years has changed 3 times faster in half the time, with the fastest change in the last 50 years.

In planetary terms, what’s happening is unprecedented. Changes of this scale and speed are unknown. Before the 19^th century, the average temperature on the planet had been around 14.5ºC for 2,000 years. By 2022 it had risen to 15.6ºC. In September 2023 it exceeded 16ºC. That is 3.5ºC higher than the average for the last million years. Without any natural event to explain what’s happening this sudden change can only be the result of human activity.

And so it is. It is no coincidence that the period of very rapid warming began at precisely the time fossil fuel use took off, in the early 19^th century. Energy, which had come from water, wind and animal power, increasingly came from burning fossil fuels. Changes in food production played a part too, as the use of nitrate-based fertilisers grew. Both led to an increase in emissions of carbon dioxide, methane and nitrous oxide, the main greenhouse gases, which was too large for nature to absorb. Humans suddenly released huge quantities of gases that had been stored for millions of years as coal, oil, gas and nitrates. These became suspended in the atmosphere, trapping some of the sun’s heat and warming the planet.

Societies can’t reduce this warming just by cutting emissions because it is the concentration of these gases in the atmosphere that is now the main problem. The concentration of CO2 in 2022 was 50% higher than it was 200 years ago.

That’s why net zero targets don’t change anything. They don’t reduce the concentration – the volume of gases that has built up in the atmosphere since the early 19^th century.

It is the concentration of gases in the atmosphere that needs to fall before the temperature can begin to drop.

For that to happen naturally would take thousands of years. The only alternative is carbon capture, to suck the gases out of the atmosphere. Even that will take decades. It will also be very expensive. There is also the problem of what to do with all the carbon that’s captured. Where and how can it be stored for thousands of years? Who will pay for that?

To understand what will happen as the temperature rises further (and it will, because further temperature increases are already locked-in ), we need to look further into the past. Looking back makes what’s happening today even clearer.

The chart below shows the average global surface temperature over 65 million years. The academic source is here. I have simplified the graph slightly. The horizontal axis is broken into different periods. The first two sections show millions of years, from 65 million years ago to half a million years ago. The middle section shows hundreds of thousands of years, from half a million years ago to 25,000 years ago. The fourth section is in thousands of years, and shows from 25,000 years ago until 1800. The last section is from 1800, the beginning of the industrial age, up to today. There is a forecast from the UN (the IPCC) to 2100.

The vertical axis shows the change in temperature in degrees centigrade (the Δ symbol means ‘change in’). It shows the change compared to the average before 1800.

The different colours of the lines relate to different scientific studies, referenced at the bottom of the chart.

You can see, for example, that the temperature 65 million years ago was 10ºC higher than the long term average before the beginning of the 19^th century.

On the far left, there’s a spike around 45 million years ago. This is believed to have been caused by an underwater volcanic eruption in what is now the Indian Ocean (the land masses were in a different place then), though the cause is not known for sure. What’s important is that there was a sudden and very large spike in temperatures, just like now. Important too, is that it took 40,000 years for the planet to correct this change. An academic discussion can be found here, on page 25. The same article has many other useful insights.

Chart 3: Global average temperature anomalies from 65 million years ago to the present day, with a forecast from the IPCC to 2100

Now look to the far right of the graph, at the red arrow, where the two dotted lines cross. This is the average global temperature in September 2023, which was 1.8ºC above the pre-industrial average. It is above the IPCC’s projections for that year, suggesting that the world is getting hotter more quickly than expected.

September 2023 had the highest average global temperature in more than 100,000 years. The last time the planet was consistently that hot was 3 million years ago, when the northern polar ice first formed. If the average temperature rises further, all the polar ice will slowly disappear. More on this below.

The ice at the South Pole, which was was formed 33 million years ago, is also at risk.

On the far right of the chart are the IPCC’s projections for the remainder of this century. At the upper end of this forecast, which is consistent with current trends, the average temperature on the planet in 2100 will be exactly as it was 40 million years ago.

That does not mean both the polar ice caps will have gone by 2100. The melting process will take many more decades. But it means that there will eventually be no ice on Earth at all. Unless, that is, efforts are made to stop what’s happening.

Even then, it’s important to understand that any attempts to stop what’s happening will take time to have an impact. In human terms, the atmospheric system is slow to change. Even if societies stopped all damaging emissions tomorrow, the temperature of the planet will continue to rise for many years.

Two other useful charts help show why small changes in temperatures are important.

The first shows the volume of sea ice in the Arctic from 1979 to 2023.

Chart 4: Arctic Ice volume, 1979 to 2023

It shows the months of the year along the horizontal axis, and the volume of ice in thousands of square kilometres on the vertical axis. The black line with dots is the average during the last 44 years. You can see there is a cycle. The volume of ice is at its highest in April and its lowest in September.

Over the last few years, the low point in September has seen 4,000 square kilometres of ice remaining. This is far below the long term average, and is a direct consequence of rising global temperatures. The average between 1979 and 2020 was 10,000 square kilometres – 2.5x more. Without radical change, the IPCC says there will be no summer ice at all by the 2030s. The North Pole will only exist in the winter. Long term, if the temperature keeps rising, it will not exist in winter either.

The final chart shows average sea surface temperatures. It shows the monthly fluctuations in the average temperature. The sea temperature is important because it greatly affects the weather – the air temperature, wind and rainfall. The life of many species depend on the sea temperature too. It is where most of the excess heat on the planet is being stored.

The middle dotted line shows the average sea temperature between 1982 to 2011. The top black line shows the temperature in 2023, from January until the start of October. The orange line is 2022.

It shows that the current sea temperature is far higher than it was 20 years ago. This partly explain the rising number and severity of weather events – floods, typhoons and droughts. As the temperature rises further, so will the frequency and severity of extreme weather.

Chart 5: Average Sea Surface temperature, 1981 to 2023

What can you do? Here’s an article about what’s likely to happen over the next few decades climate-wise, why it is happening, and what you can do.

If you have any questions or comments please email me.