Our historical emissions

The atmosphere is currently burdened with an excess of greenhouse gases (GHGs), particularly carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These gases are the primary drivers of climate change, leading to rising global temperatures. Human activities, such as burning fossil fuels, deforestation, and industrial processes, have increased the concentrations of these gases to levels unprecedented in human history. Understanding the magnitude of these emissions and outlining possible scenarios for reducing them to meet global temperature goals is critical for addressing the climate crisis.

The Historical Emission of Excess Carbon and Other Greenhouse Gases

Since the Industrial Revolution in the late 18th century, the atmospheric concentration of CO2 has increased dramatically due to human activity. According to the Intergovernmental Panel on Climate Change (IPCC), the pre-industrial concentration of CO2 in the atmosphere was around 280 parts per million (ppm). As of 2021, this figure stands at over 410 ppm, primarily driven by fossil fuel combustion and deforestation.

Cumulative CO2 emissions between 1850 and 2019 are estimated to be approximately 2,400 gigatons of CO2 (GtCO2), with about 1,650 GtCO2 of these emissions resulting from fossil fuel combustion and cement production, and the remaining 750 GtCO2 from land use change, primarily deforestation. For context, a gigaton (Gt) is one billion metric tons, and the current global annual emissions of CO2 are about 40 GtCO2, a figure that continues to rise annually.

In addition to CO2, other GHGs like methane (CH4) and nitrous oxide (N2O) have seen significant increases due to human activities. Methane, for example, is over 25 times more potent as a GHG than CO2 over a 100-year period, and its atmospheric concentration has increased from about 722 parts per billion (ppb) in pre-industrial times to around 1,880 ppb today. Nitrous oxide, another potent GHG, has increased from around 270 ppb to over 330 ppb, primarily due to agricultural practices.

The Global Warming Scenarios: 1.5°C, 2°C, and 3°C

The global community has rallied around specific temperature targets to limit global warming. These targets—1.5°C, 2°C, and 3°C—are based on the levels of global temperature rise above pre-industrial levels. Each scenario represents a different path of GHG emissions and has vastly different implications for the planet.

1.5°C Scenario

Limiting global warming to 1.5°C is considered the most ambitious target and is consistent with the Paris Agreement. According to the IPCC’s Special Report on Global Warming of 1.5°C, to achieve this goal, global CO2 emissions must be reduced by approximately 45% from 2010 levels by 2030, reaching net zero by 2050. This would require a reduction of around 20 GtCO2 annually by 2030, and the use of negative emissions technologies like carbon capture and storage (CCS) to remove excess CO2 from the atmosphere.

In this scenario, the most severe impacts of climate change could be avoided. For example, extreme weather events like heatwaves, droughts, and floods would still increase, but at a much lower rate. Sea-level rise would be reduced, limiting the exposure of coastal regions to flooding and erosion. Coral reefs, which are highly sensitive to temperature changes, would have a better chance of survival, although significant damage would still occur. The Arctic would still experience ice-free summers, but the frequency would be much lower compared to higher temperature scenarios. Additionally, this scenario could reduce the risk of crossing climate tipping points, such as the irreversible melting of polar ice caps or the collapse of the Amazon rainforest.

2°C Scenario

The 2°C scenario is often cited as the upper limit of "safe" warming, although it still comes with significant risks. To limit warming to 2°C, the world would need to reduce CO2 emissions by about 25% from 2010 levels by 2030, and achieve net-zero emissions around 2070. This would require annual CO2 reductions of about 15 GtCO2 by 2030. The use of negative emissions technologies would still be necessary, although the demand for these technologies would be less than in the 1.5°C scenario.

In the 2°C world, the risks of climate change are significantly higher. Extreme weather events would become more frequent and intense, and the risk of droughts, floods, and heatwaves would increase. Sea levels are projected to rise by approximately 0.5 to 1 meter by 2100, which would put hundreds of millions of people at risk, particularly in low-lying coastal areas. Coral reefs would almost entirely disappear, and the Arctic would experience ice-free summers more frequently, with potentially devastating impacts on wildlife.

The 2°C scenario also increases the likelihood of triggering tipping points. For example, the Greenland ice sheet could reach a point where its melting becomes irreversible, contributing significantly to sea-level rise. Other ecosystems, like the Amazon rainforest, may also be pushed to the brink of collapse, further accelerating global warming.

3°C Scenario

In a 3°C warming scenario, the world faces far more severe consequences. Without substantial reductions in GHG emissions, the planet is currently on track to exceed 3°C of warming by the end of the century. In this scenario, global CO2 emissions would continue to rise, potentially reaching around 50-55 GtCO2 annually by 2050. This would lead to catastrophic impacts on ecosystems, economies, and human health.

At 3°C of warming, extreme weather events would become the norm, with devastating consequences for agriculture, infrastructure, and human lives. Prolonged droughts, intense storms, and frequent flooding would lead to widespread displacement of people and significant economic losses. Sea levels could rise by over 1 meter by 2100, submerging major coastal cities like New York, Miami, and Tokyo, and causing mass displacement of populations in vulnerable regions such as Bangladesh and the Pacific Islands.

Ecosystems would be severely damaged, with many species facing extinction due to habitat loss and changing climatic conditions. Coral reefs would be virtually eliminated, and the Arctic would likely experience ice-free summers almost every year. The risk of crossing tipping points would be extremely high, with irreversible changes to the Earth’s systems, such as the collapse of the West Antarctic ice sheet or the shutdown of major ocean currents like the Atlantic Meridional Overturning Circulation (AMOC).

Reducing Emissions: Technological and Policy Solutions

To achieve the 1.5°C or even 2°C scenarios, a combination of technological innovation, policy changes, and societal shifts will be necessary. The most important step is to drastically reduce the use of fossil fuels and transition to renewable energy sources like solar, wind, and hydropower. Electrifying the transport sector, improving energy efficiency, and reducing emissions from industries such as cement and steel are also key.

Negative emissions technologies, such as afforestation, reforestation, and direct air capture (DAC), will play a crucial role in removing excess CO2 from the atmosphere. However, these technologies are still in the early stages of development and need significant investment to scale up.

International cooperation will be essential in achieving global emission reductions. Policies like carbon pricing, subsidies for renewable energy, and the phasing out of fossil fuel subsidies can help drive the transition to a low-carbon economy. In addition, efforts to protect and restore ecosystems that act as carbon sinks, such as forests, wetlands, and mangroves, are crucial for mitigating climate change.

Conclusion

The amount of excess GHGs in the atmosphere has already set the world on a trajectory toward dangerous levels of global warming. The decisions made in the next decade will determine whether we can limit warming to 1.5°C, 2°C, or face the dire consequences of 3°C and beyond. Achieving the 1.5°C or 2°C targets requires immediate, large-scale reductions in emissions and significant changes in how energy is produced, consumed, and managed.

Sources:

1. IPCC Special Report on Global Warming of 1.5°C (2018)

2. Global Carbon Project (2021) "Global Carbon Budget 2021"

3. NOAA (National Oceanic and Atmospheric Administration) Climate Data

4. NASA GISS (Goddard Institute for Space Studies) Climate Change Data