How Do We Study Past Climate?

One of the ways scientists try to understand climate change today is by understanding climate change in the past.  Scientists study a range of organisms and other features to learn how the climate has changed in the past, better understand relationships between greenhouse gases and temperature, and learn how species responded to past climate change.

Humans weren't around measuring the temperature with thermometers or measuring CO2 tens or hundreds of thousands of years.  In order to estimate what the climate and greenhouse gas concentrations were in the ancient past, scientists measure proxies.  When you can't measure something directly, you measure a proxy. A proxy is something you can measure that allows you to indirectly measure the variable of interest.  For example, if we want to measure the temperature of the geologic past, we can measure the amount of pollen from plants that like warm temperatures. The pollen is a proxy for temperature.

Proxies 

Below are four examples of how scientists study past climate change to better understand climate change today.  Watch the videos below.  

1. Tree Rings

2. Coral Cores

3. Sediment Cores for Pollen

4. Ice Cores

Ice cores are special.  Ice cores can tell us not just how the climate has changed, but can also tell us what the composition of the atmosphere was when that ice layer was formed because gases become trapped in air bubbles in the ice.

What Do These Studies Tell Us?

The studies of different proxies all tell a similar story - temperature and greenhouse gas concentrations are highly correlated.  When greenhouse gas concentrations were high in the past, so were temperatures.  When greenhouse gas concentrations were low in the past, so were temperatures.  

The graph below shows the relationship between CO2 and temperature going back over 400,000 years.  

• • The CO2 concentration, on 3/17/2023 , was 420.32 ppm. This is higher than at any point in at least the last 800,000 years.   
  • It has increased by 0.32% in the last year
  • You can check the current concentration at this website:  CO2 Earth