The road to our future climate: How are CO2 emissions affecting the Earth? Which climate action are you ready to take?
Carbon dioxide (CO2) emitted by human activities is changing the Earth’s climate, increasing the atmospheric CO2 concentration and raising global surface temperatures. While world leaders have committed to taking action to limit global warming well below 2°C through the Paris Agreement , current policies fall far short of these goals. It is critical to understand the effect of different mitigation policies on the future climate to inform policymakers and appropriately design mitigation and adaptation strategies.
In the 4C project, we work to further our understanding on how the emitted carbon interacts with the Earth’s system (i.e. the global carbon cycle ), and to improve near-term predictions and long-term projections based on policy scenarios. Here, we explore the CO2 and temperature evolution in the past, present and future in an attempt to shed light into the expected changes and encourage further action.
While humans have deforested land and burnt coal for hundreds of years, the industrial revolution led to a sharp increase in such activities. Burning fossil fuels, deforestation and changes in land use, among others, are emitting CO2 into the atmosphere, increasing the atmospheric CO2 concentration.
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CO2 emissions differ in each country and come from different sectors (such as transportation, energy production and industry). The main sources include coal, natural gas or oil burning, cement production and other sources, like flaring and bunker emissions.
In 2021 , global fossil CO2 emissions reached 36.6 billion tonnes CO2, likely a new record high (still to be confirmed given uncertainties). The tree ring visualisations below depict the growth of each emission source in the top four emitting countries across the years.
Atmospheric CO₂ levels vary not only because of human activities, but also vary on a seasonal timescale due to natural variability . Distinguishing between natural and human-induced changes in CO2 concentration is crucial for policy making. It can help us understand if changes seen in recent years were due to measures taken or because of natural variations in CO2, in which case further action is needed.
Another important concept for policymaking is the remaining carbon budget , which is calculated annually and shows how much carbon we can emit until we exceed a temperature goal (data shown for 2022; source: Global Carbon Budget).
Remaining CO2 Budget
Accurate calculations of the future evolution of carbon sources and sinks are important for determining the remaining carbon budget. Sources include the combustion of coal, oil and gas, and deforestation, which release CO2 into the atmosphere. Sinks include the uptake of carbon in the ocean and land. These sinks partially take up the CO2 emitted into the atmosphere by sources; however, a large proportion of this emitted CO2 remains in the atmosphere.
The amount released by sources should theoretically be equivalent to that absorbed by sinks, although in practice a budget imbalance is seen, due to uncertainties in our data estimates and ability of models to fully replicate internal variability of the climate system. Research to reduce this budget imbalance and uncertainties is undergoing and important for future policies.
At the moment, about 45% of the emitted CO2 remains in the atmosphere and raises the atmospheric CO2 concentration, while the rest (55%) is absorbed by the land and ocean (carbon sinks). In turn, the rising atmospheric CO2 increases the global temperature, causing changes and damage to the environment: ice caps are melting, sea levels are rising, and extreme weather events like heatwaves, droughts and floods become more frequent.
By better understanding the variations of CO2 and temperature in the past, we can improve the Earth System models that predict these and consequently make better predictions of future changes, to inform climate policymaking.
Given the current policies, CO2 emissions are expected to continue increasing during this decade (~2030) before levelling out. More policy ambition is needed to ensure that the emissions peak, and then decline sufficiently fast. The pace of this increase and the timing of peak emissions depends on the measures adopted by countries to mitigate climate change. At the same time, as long as we emit CO2, the CO2 concentration and temperature will also keep rising.
The increase in CO2-induced global warming will only stop when humans stop adding CO2 to the atmosphere . Nevertheless, the pathway for reaching net zero emissions is important when dictating the climate strategies to follow.
To stabilise the Earth's system and temperature, urgent action is necessary. Taking action earlier rather than continuing to increase emissions in the meantime would mean that we prevent additional warming, avoiding the potentially devastating effects of climate change.