Climate Projection Table

Figure 1.2 demonstrates a strong correlation between [CO2] and ΔT at the rate of 0.010oC/ppm CO2. At this rate a doubling of CO2 concentration from 285 to 570 will result in a 2.85oC rise in ΔT. As mentioned earlier, Svante Arrhenius calculated that doubling CO2 would result in an increase in Earth’s average surface temperature of approximately 5–6°C. Melting ice is the likely explanation for this discrepancy. In Slater et al., 2021, Earth’s ice imbalance 4 it is estimated ~0.028 Tt (trillion tonnes) of ice have been lost since 1970. This would have consumed enough latent heat to increase ΔT 1.83oC which accounts for much of the dfference between Arrhenius’s 6oC and the projected 2.85oC from Figure 1.2. Fortunately, the ice lost to date is only a tiny fraction of the whole ~27.2 Tt (trillion tonnes) so the linear relationship between CO2
concentration and global temperatures should endure until there are practical ways to rein in the use of fossil fuels for manufacturing, power generation, agriculture, transport and air-conditioning.

The Paris Agreement signed by delegates to the 21st Conference of the Parties (COP21) to the United Nations Framework Convention on Climate Change (UNFCCC) in Paris in December 2015 provides targets for this endeavour. The agreement binds the 197 signatories to the agreement

• to limit global temperature increase to well below 2 degrees Celsius above preindustrial levels.
• to pursue efforts to limit the temperature increase to 1.5 degrees Celsius above pre-industrial levels.
• to reduce greenhouse gas emissions to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century (net zero emissions).
• to prepare, communicate and maintain successive Nationally Determined Contributions (NDCs) that individual countries intend to achieve.

As a basic tool to plan and monitor these fforts on a global scale, Figure 1.3 is a spreadsheet analysis of historical records that allows them to be projected forward from 2023. The underlying principle for the analysis of past records and the forward projections is that global warming is caused by the atmospheric greenhouse effect. CO₂ is a greenhouse gas. It absorbs and re-emits infrared radiation, trapping heat in the Earth’s atmosphere. Excess CO₂ traps more heat disrupting the balance between incoming solar radiation and outgoing infrared radiation. This energy imbalance causes global surface warming, ocean heat uptake, melting ice sheets, rising sea levels and changes in weather patterns. Each of these effects can be observed, the most obvious being global surface warming (see Figure 1.2) and melting ice sheets.

Between the end of the last Ice Age (~11,700 years ago) and the start of the industrial revolution, say 1850, Earth’s climate was stable save for minor natural fluctuations. [CO2] was 285 ppm equivalent to a baseline atmospheric CO2 quantity, Q0 of 2.229 trillion tonnes. ΔT was zero. The first step in analysing historical carbon dioxide emission destinations from that time on is the creation of an accurate historical record of CO2 emissions.

This involves a combination of statistical reporting, fuel consumption data, industrial activity records, and satellite measurements. The process begins with national and sectoral data on fossil fuel use coal, oil, and natural gas— ollected by governments and energy agencies. Emissions from cement production, land use changes, and deforestation are also estimated using activity data and scientific models. These figures are then converted into carbon dioxide equivalents using standardized emissions factors. Independent scientific institutions, such as the Global Carbon Project, aggregate and verify this information to produce consistent global estimates. Increasingly, satellite-based atmospheric observations and machine learning techniques are used to cross-check and refine the accuracy of reported emissions, particularly in regions with limited transparency. The result is a comprehensive yearly snapshot of how much CO₂ humanity adds to the atmosphere. The record adopted for the compilation of Figure 1.3, ‘Global temperature anomaly beyond 2024 based on projected global CO2 emissions’ is published by Our World in Data5.

There is now sufficient data to compile the table up to 2023. Residual CO2 in the atmosphere in trillion tonnes, Qr, is simply [CO2] multiplied by 7.821/1000. Excess CO2 in the atmosphere in billion tonnes is (Qr-Q0)*1000. The annual residual CO2 increase in the atmosphere ΔQ is simply the increase in Qr from one year to the next in billion tonnes.

The CO2 emissions that do not remain in the atmosphere are absorbed by land and sea in what are known as the terrestrial and marine carbon sinks. Plants absorb CO₂ from the air and use it to produce sugars via photosynthesis. Forests, grasslands, and crops are major carbon absorbers, especially tropical and temperate forests. CO₂ from the atmosphere dissolves into the ocean surface. Tiny plankton in the ocean’s surface absorb CO₂ during photosynthesis. When these organisms die, the carbon in their bodies sinks into the deep ocean, sequestering it for centuries or more. CO₂ reacts with seawater to form bicarbonate (HCO₃⁻) and carbonate ions (CO₃²⁻). This buffering system allows the ocean to store vast amounts of carbon although it also leads to ocean acidification.

QLS, the CO2 absorbed by land and sea is simply the difference between annual emissions Qe and the annual residual CO2 increase in the atmosphere ΔQ. The ice records show that [CO2] began to grow around 1880 when it had reached 290 ppm. An examination of the historical records show that it took around 100 years for the carbon dioxide cycle to stabilise but when it did it became clear that the annual land sea absorption rate fell into a band between 1.3 and 2.4% of the excess CO2 in the atmosphere (Qr-Q0) with a mean of 1.83% as per Figure 1.3.