Historical climate change
Understanding rising global temperatures in the context of Earth’s history
The world’s climate has changed throughout its history and has even been warmer than today, so what caused this and why is it different from the present rise in global temperatures?
Historically, natural changes in the Earth’s orbit cause global temperature change over the course of thousands, rather than hundreds of years. Gradual shifts in Earth’s orbit and tilt (Milankovitch cycles) alter its seasons and distance to the sun – a change which can last tens of thousands of years – creating cycles of warm (interglacial) and cold (glacial) episodes. These orbital variations result in changes to both global temperature and atmospheric carbon dioxide. Palaeoclimate research has shown that when carbon dioxide increases, the global mean temperature increases, and when carbon dioxide falls global mean temperature falls.
Today, the Earth remains in a (warmer) interglacial period called the Holocene and has done for around the last 12,000 years, but global temperature rise over the last 100 years is happening at a rate 10 times faster than the average rate of warming after an ice age, surpassing any point during the last 2,000 years1. This rise in global temperatures is being driven by carbon dioxide being released by humans at a rate 250 times faster than from natural sources after the last glacial period.
The climate misinformation you may encounter
- Temperature has fluctuated a lot in the past 2,000 years without any human influence. Today’s temperature change is just another of these fluctuations.
Our response: Global climate has remained relatively stable over the last 11,000 years, supporting human civilisation’s development, until the last 150 years when humans began releasing large quantities of carbon dioxide into the atmosphere (Kaufman et al. 2020). Periods such as the ‘Roman Warm Period’, the ‘Medieval Warm Period’ and the ‘Little Ice Age’ are examples during the past 2,000 years where warmer temperatures occurred in some regions of the world (Neukom et al. 2019). Today’s rapid warming is happening across the globe due to human activities releasing carbon dioxide into the atmosphere at a far greater rate than natural processes.
Source: IPCC AR6 WG1: | Global surface temperature as estimated from proxy records (reconstructed) and climate models (simulated). The intent of this figure is to show the agreement between observations and models of global temperatures during palaeo reference periods. (a) For individual palaeoclimate reference periods. (b) For the last millennium, with instrumental temperature (AR6 assessed mean, 10-year smoothed). Model uncertainties in (a) and (b) are 5–95% ranges of multi-model ensemble means; reconstructed uncertainties are 5–95% ranges (medium confidence) of (a) midpoints and (b) multi-method ensemble median. {2.3.1.1, Figure 2.34, Figure 3.2c, Figure 3.44}
- Can we trust temperature records from thousands of years ago given they cannot be measured directly?
Our response: Yes, we can trust these records. Scientists have made robust estimations of past climates using ‘proxies,’ which are natural archives of past climates, such as ice cores, tree rings, coral skeletons, or sediment layers. These proxies record some physical or chemical property that changes with the climate, like the chemical composition of a sediment that reacts to temperature variations. By understanding the relationship between modern-day proxies and temperatures, researchers can analyse ancient records of them and make inferences about prehistoric climates. There are more than 10,000 proxy studies worldwide, collectively painting a picture of Earth's climate before we had modern temperature records. Scientists can also combine different proxy records of different types and from different locations. Looking for where their estimates overlap gives us more precise estimates of past temperatures, helping filter out local variations and differences in how proxy records measure temperature. If you're curious to explore further, you can find more information at the NOAA Paleoclimatology website.
- Do we need a warmer planet to stop us going into an ice age?
Our response: No. The next ice age isn’t expected to happen for at least another 50,000 years, according to long-term projections of the Earth’s orbit. The more immediate problem is limiting the worst impacts of human-caused climate change. This is already costing lives around the globe through extreme weather events which will become more frequent as global temperatures rise. We can limit the most damaging impacts of climate change by reducing and removing our emissions of carbon dioxide from the atmosphere.
- It was warmer in the past so today’s climate change is nothing to worry about.
Our response: The Earth is currently warming abnormally quickly2 due to human-released carbon dioxide. While it is true that Earth has experienced comparably rapid and large changes in temperatures in the past few hundred million years, current climate change is particularly dangerous because modern animals and plants (including humans) have not experienced changes of this scale and speed before, making it challenging for us (and the biodiversity we rely on) to adapt3,4. By 2100, Earth’s average temperature is projected to be roughly 3.2 °C above the pre-industrial levels which is the hottest period in at least the last 3 million years3.
- Carbon dioxide isn’t responsible for the warming. It was warmer in the past when carbon dioxide levels were lower.
Our response: Atmospheric carbon dioxide and global temperatures are closely intertwined. During the last interglacial (which began 115,000 years ago), temperature increases drove atmospheric carbon dioxide levels up; it may even have been a few degrees warmer than today despite having less carbon dioxide in the atmosphere5 because the Earth was tilted towards the sun and received more heat in the northern hemisphere. This mechanism for driving global temperatures is different to today where instead, the carbon dioxide released since the pre-industrial period (rather than a slow orbital cycle), is driving global temperature rises at a rate 10 times faster than the last interglacial.
1 https://www.un.org/en/climatechange/what-is-climate-change
2 https://www.pnas.org/doi/pdf/10.1073/pnas.1809600115
3 https://climatescience.org/advanced-climate-future-temperatures
4 https://www.pnas.org/doi/pdf/10.1073/pnas.0913352107
5 https://www.nature.com/articles/nature19798