Atmospheric Carbon Dioxide and Climate Feedbacks
Carbon Dioxide: Earth’s Climate Thermostat
Atmospheric carbon dioxide (CO2) is one of the most powerful controls on global climate over geological timescales. It acts as a greenhouse gas — it allows incoming solar radiation to reach the surface but absorbs outgoing infrared radiation, warming the lower atmosphere. When CO2 concentrations are high, the planet warms; when they fall, the planet cools. Understanding where CO2 comes from, how its concentration has changed through time, and what feedback mechanisms amplify or dampen those changes is essential for GCSE Geology.
Sources of Atmospheric CO2
CO2 enters the atmosphere through two main geological pathways. The first is volcanic emissions. Volcanoes release CO2 stored in the mantle whenever magma degasses at the surface or at mid-ocean ridges. Over geological timescales of millions of years, the total rate of volcanism — controlled by how active plate tectonics is — determines the background level of atmospheric CO2. Periods of intense volcanic activity (such as the eruption of large igneous provinces like the Deccan Traps, about 66 million years ago) can significantly increase atmospheric CO2 and warm the climate.
The second pathway, operating on a much shorter human timescale, is the burning of fossil fuels. When coal, oil and natural gas are burned, carbon that was locked in rocks for hundreds of millions of years is rapidly returned to the atmosphere. The current rate of CO2 increase from fossil fuel burning is far faster than any volcanic source — approximately 100 times greater than global volcanic emissions — which is why atmospheric CO2 concentrations have risen from about 280 parts per million (ppm) before industrialisation to over 420 ppm today.
The two sources of atmospheric CO2 are volcanic emissions (geological timescale — millions of years) and the burning of fossil fuels (human timescale — decades to centuries). Both release carbon that was previously stored, but at very different rates. Current fossil fuel burning raises CO2 concentrations far faster than any volcanic source.
Evidence from Ice Cores
How do geologists know what atmospheric CO2 concentrations were in the past? One of the most direct pieces of evidence comes from ice cores. Ice cores drilled from the Antarctic ice sheet contain air bubbles trapped when snowflakes compressed into ice hundreds of thousands of years ago. Analysing these bubbles allows scientists to measure the actual CO2 concentration of the ancient atmosphere directly. The oldest Antarctic ice cores extend back approximately 800,000 years and show that CO2 has fluctuated between roughly 180 ppm during ice ages and 280 ppm during warm interglacial periods — closely tracking global temperature as recorded by oxygen isotope ratios in the same ice.
Ice cores also contain other climate proxies. The ratio of oxygen isotopes (oxygen-18 to oxygen-16) in the ice itself records past temperature — lighter oxygen-16 evaporates preferentially from the ocean, and colder periods preserve more of it in polar ice. Dust layers in ice cores record periods of intense wind (often corresponding to cold, dry glacial episodes). Together, these proxies make ice cores one of the most information-rich climate archives available, though they only reach back 800,000 years — a tiny fraction of Earth’s history.
For periods beyond 800,000 years, geologists cannot use ice cores (no ice survives from before the Quaternary in Antarctica). Instead, they use proxy evidence from sediment cores, the carbon isotope ratios preserved in fossil shells, and the stomatal density of fossil leaves (leaves grow fewer stomata when CO2 is high). The sedimentary rock record also preserves indirect evidence — for example, the vast coal measures of the Carboniferous formed from organic carbon buried at an enormous rate, which may have drawn down CO2 and helped trigger the Carboniferous–Permian glaciation.
Positive and Negative Feedbacks
A feedback mechanism is a process that either amplifies or dampens an initial climate change. Understanding feedbacks is crucial because they determine whether a small change in CO2 leads to a small or a dramatic shift in global temperature.
A positive feedback amplifies the initial change. The ice–albedo feedback is a key example: when global temperature rises, ice melts; the dark ocean or land surface exposed has a lower albedo than ice (absorbs more radiation instead of reflecting it); this causes further warming; which melts more ice; amplifying the original warming. The water vapour feedback is similar: warming increases atmospheric water vapour, which is itself a greenhouse gas, causing further warming.
The ice–albedo positive feedback: (1) warming melts ice and snow; (2) the dark ocean or soil exposed absorbs more solar radiation than the reflective ice did; (3) this causes more warming, which melts more ice — the process reinforces itself. This is why relatively small initial changes in CO2 can lead to large shifts in global temperature.
Negative Feedbacks: Nature’s Thermostats
A negative feedback counteracts the initial change and helps stabilise the climate. The most important geological negative feedback is chemical weathering. When the climate warms, the rate of chemical weathering of silicate rocks increases (warmer temperatures and more rainfall accelerate reactions). This removes CO2 from the atmosphere (CO2 dissolves in rainwater to form carbonic acid, which reacts with silicate minerals, eventually depositing carbonate in the ocean). The reduced atmospheric CO2 then causes cooling — opposing the initial warming. Over millions of years, this “weathering thermostat” has helped prevent CO2 from rising to catastrophic levels.
Other negative feedbacks include marine storage (as CO2 rises, more dissolves in ocean water and is used by organisms to build shells, which eventually sink to the seafloor as carbonate sediment) and, over very long timescales, subduction of carbon-rich ocean crust, which returns carbon to the mantle and ultimately recycles it through volcanism over tens of millions of years. Modern engineering efforts to mimic these natural processes — known as carbon capture and sequestration — draw directly on this geological understanding.
It is easy to confuse the terms. Remember: “positive” does not mean “good” — it means the feedback amplifies the change. “Negative” does not mean “bad” — it means the feedback reduces the change and acts as a stabiliser. Exam questions often give a scenario and ask you to identify whether a feedback is positive or negative. Ask yourself: does the feedback make the initial change bigger (positive) or smaller (negative)? The ice–albedo loop makes warming bigger (positive). Chemical weathering makes warming smaller (negative). Apply that test to any scenario.
Connecting the Evidence
The ice core record, the sedimentary rock record, and our understanding of feedback mechanisms all fit together into a consistent picture: atmospheric CO2 is a powerful climate driver, but its effects are mediated by feedbacks that can either amplify small changes into major climate shifts (positive) or moderate those changes over geological time (negative). The current rapid rise in CO2 from fossil fuels is outpacing the slow negative feedbacks — particularly chemical weathering, which operates over hundreds of thousands of years — which is why human-induced climate change is proceeding faster than any natural climate cycle in the ice core record.