What is the Evidence for Climate Change?

Tom Haddon

Tom Haddon

07 Oct, 2020

What is the Evidence for Climate Change?

Climate change is very real. We can see it in the changing seasons, the unusually high temperatures and the increase in natural disasters. As well as what we can see ourselves, we are constantly told by scientists that climate change is here, and it is very dangerous. 

However, what we aren’t usually told about is the specific pieces of evidence that allow scientists to be so sure of their estimates. It’s also very hard for many to see where statements such as

temperatures are hotter than they have been for millenia

come from. This article will run through the different evidence for climate change, and the methods scientists use to find data on temperatures from hundreds of years ago, describing and evaluating each method for their trustworthiness.


Global temperatures

There has been an undisputed rise in global temperatures over the past 150 years,

due to increasing carbon dioxide and human-caused greenhouse gases in the atmosphere. 2023 was the warmest year on record, due to human-induced climate change and the natural El Nino weather event.

2023 was about 1.48 degrees C warmer

than pre-industrial levels, practically hitting the 1.5 degrees target.

Glacier retreat 

Across the world, glaciers are retreating, and shrinking in size. This can be recorded and tracked over time to show how the Earth is heating up. It’s not just glaciers, ice sheets in Greenland and the Antarctic are losing unprecedented amounts of ice. Additionally, looking at a longer time scale, we can see that there are ‘fossil glacier landscapes’ which are areas that once would have contained glaciers, but due to warming these glaciers are no longer there.

Glacier retreat also results in multiple complex positive and negative feedback mechanisms. One example is the positive ice-albedo feedback. When glaciers melt, this increases the area of dark oceans and decreases the area of white snow consequently lowering the albedo. This means less solar radiation is reflected back to space, leading to rising ocean temperatures and further glacial melting. 

Sea level change

Sea level change is one of the most cited pieces of evidence for climate change. Globally, sea levels have risen by

over 8 inches

over the last century. Sea level rose for two main reasons: firstly the melting of ice on land and secondly the thermal expansion of water as it heats up. As has been explained above, glacier retreat and ice melting are happening at unprecedented levels, but sea temperatures are also rising alongside this.

In 2023, global ocean temperatures were

0.1 degrees Celsius above 2022


Vatnajökull National Park, Iceland. © L.W. on Unsplash

Vatnajökull National Park, Iceland. © L.W. on Unsplash

Ocean Acidification

Ocean water is not acidic naturally. Although slightly salty, the natural pH level of the ocean is actually closer to being alkaline than acidic. However with increasing carbon dioxide in the atmosphere, the water is becoming increasingly acidic. In fact, since the beginning of the industrial revolution, the pH of the ocean has fallen by approximately 0.1 units which equates to around a

30% increase in ocean acidity

. This is because the ocean is a large carbon sink – in fact it is estimated that the ocean stores up to

60 times

more carbon than the atmosphere. This storage of carbon is one of the main reasons why the ocean is turning more acidic. 

Methods of analysis

Ice core analysis

As snow falls it traps air, which over hundreds of years is compressed forming tiny bubbles of oxygen and CO2 within ice sheets. Ice core analysis involves drilling down into the ice and examining a rod of ice that is removed, with each layer of compressed snowfall representing a year. The temperature can be calculated through the ratio of carbon dioxide compared to oxygen, with a higher ratio of CO2 meaning a hotter climate. This is an extremely reliable method of analysing Earth temperatures, which can be compared to current figures. 


This method is similar to ice core analysis. If you cut a bisection of a tree you will be able to see rings, each of which represents a year’s worth of growth. A tree will grow more in a warm, wet year and less in a cold year, so it is possible to  track the warming of a climate through these tree rings. The best trees to analyse for this method are the giant Californian redwoods, since they are older than most other species of tree and have a very regular trunk structure. This is a useful measure of climate change especially over the last 200 years, however, one of the main problems with this method is that other factors can affect the rings in the trees, such as soil richness, and precipitation levels.

Sea floor sediment analysis

This is a very accurate method of measuring climate change and gives scientists some of the best long term data. Put simply scientists can look at the material that settles to the bottom  of the ocean – known as sediment. By analysing this, they can see the different chemical variants of oxygen – known as isotopes – which are different because of temperature or pressure. By looking at the ratio between these different kinds of settled oxygen we can determine climate history.  O16 isotopes of oxygen are lighter than O18 ones so therefore evaporate more readily. Hence layers of sediment with comparatively higher levels of O18 signify warmer periods since more O16 will have evaporated. This can also be used in conjunction with carbon dating (explained next) to create very accurate results.

Carbon dating

Carbon dating also involves carbon isotopes, which, more simply put, are different variants of the chemical carbon. By using the isotope of carbon C-14 which decays at a known rate, and comparing this to the isotope C-12 which does not decay, it is possible to find the age of plant matter. This method can tell us the ages of plants up to 50,000 years old. From here, the amount and distribution of plants can be found which indicates the climate at the time.

Extreme weather

You may not think of it as evidence for climate change but extreme weather events are actually very indicative of a changing climate. Extreme heat and increasing droughts are clearly caused from rising global temperatures. However, one result of this is that due to high levels of evaporation there is more moisture in the atmosphere, and so it is expected that we will see more category 4 and 5 storms as temperatures continue to rise. In some scenarios such as with tropical revolution storms (more commonly known as a typhoon, cyclone or hurricane), there needs to be quite specific conditions for them to form. For these tropical storms, the sea temperature needs to be 26 degrees celsius or higher, which, with increasing global land and sea temperatures, is becoming more and more common.

An isolated single cell thunderstorm with an amazing light show © NOAA on Unsplash

An isolated single cell thunderstorm with an amazing light show © NOAA on Unsplash