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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.
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. In fact over the last 35 years, higher global temperatures have been particularly noticeable with 16 of the 17 warmest years on record occurring since 2001 . In 2016, globally average temperatures were 0.99 degrees celsius warmer than the mid-20th century average  – worryingly close to the 1.5 degrees celsius target to stay below.
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.
Sea level change
Sea level change is one of the most cited pieces of evidence for climate change. Globally, sea levels rose by 8 inches over the last century . Sea level rise 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. Since 1969 the top 100m of the oceans across the board have increased 0.6 degrees Fahrenheit .
Vatnajökull National Park, Iceland. © L.W. on Unsplash
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 50 times more carbon than the atmosphere . This storage of carbon is one of the main reasons why the ocean is turning more acidic.
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 analyze 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 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.
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
 Kunzemann. (2010). How do we know that the planet is hotter than ever before? https://www.allianz.com/en/press/extra/knowledge/environment/100812-how-do-we-know-the-planet-is-hotter-than-ever-before.html
 NASA. (2017). NASA, NOAA Date Shows 2016 Warmest Year on Record Globally. https://www.giss.nasa.gov/research/news/20170118/
 NASA. (2018). Climate Change: How Do We Know? https://climate.nasa.gov/evidence/
 Levitus et al. (2017).
NCEI ocean heat content, temperature anomalies, salinity anomalies, thermosteric sea level anomalies, halosteric sea level anomalies, and total steric sea level anomalies from 1955 to present calculated from in situ oceanographic subsurface profile data. https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc:0164586
 NOAA. (2020). What is Ocean Acidification? https://www.pmel.noaa.gov/co2/story/What+is+Ocean+Acidification%3F
 Ocean and Climate Platform. (2020). The Ocean, A Carbon Sink. https://ocean-climate.org