10 November 2023
James Hansen, Pushker Kharecha, Norman Loeb, Makiko Sato, Leon Simons, George Tselioudis, and Karina von Schuckmann

The drive for global temperature change is Earth’s energy imbalance (EEI), the difference between the energy Earth receives from the Sun and energy Earth reflects and radiates back to space. We have good measurement of EEI today based on precise satellite data for change of reflected and emitted radiation calibrated by decadal ocean heat content change measured by deep-diving Argo floats. Interpretation of global temperature change and prediction of future temperature requires knowledge of the principal forcings that now affect EEI: human-made greenhouse gases (GHGs) and atmospheric aerosols (fine airborne particles). Aerosol climate forcing is not being measured, but information on aerosol forcing can be extracted from an ongoing “great inadvertent aerosol experiment” as a result of discrete changes in International Maritime Organization (IMO) regulations on the sulfur content of ship fuels. These limited assessment tools are threatened by the absence of firm plans to continue direct EEI observations. A shortcoming of our climate science is failure to communicate well what is known from existing data. Global warming in the pipeline and emissions in the pipeline assure that the goal of the Paris Agreement – to keep global warming well below 2°C – is already dead, if policy is constrained only to emission reductions plus uncertain and unproven CO₂ removal methods.

Delayed response of climate makes human-made climate change a grave threat, especially for young people. Governments will not make required changes to energy policies based on theoretical threats – there must be sufficient empirical evidence of harm to force action. Thus, delayed response makes it difficult to avoid near-term, growing, climate impacts, but it does not prevent achievement of policies that will lead to a hospitable climate with a bright future for young people. Time is running short, however, and effective actions at this point require a good understanding of ongoing climate change and the responsible mechanisms.

The proximate cause of ongoing global warming is Earth’s energy imbalance (EEI). Earth is now absorbing more energy incoming from the Sun than the planet is sending back to space as reflected solar light and emitted thermal (heat) radiation. As long as that imbalance is positive – more energy coming in than going out – Earth will continue to get hotter. Factors that alter Earth’s energy balance are called climate forcings. There are two large human-made climate forcings: changes of atmospheric greenhouse gases (GHGs) and changes of aerosols. GHGs reduce heat radiation to space; thus, an increase of GHGs causes a positive energy imbalance, more energy coming in than going out, which causes warming. Aerosols reflect sunlight to space, which is a negative contribution to EEI that causes cooling.

Greenhouse Gases. GHGs are accurately measured and their total climate forcing can be calculated with an error less than or about 10%. GHG forcing increase since 1750 is about 4.1 W/m²; as shown in Fig. 1 of our Pipeline paper,¹ our calculation agrees well with that of the most recent IPCC report.² We can even calculate the annual change of the GHG climate forcing to high precision. In our graph of this annual change (Fig. 1), we show the five-year running mean because the large amount of “noise” in the annual mean tends to hide the trends that we wish to understand.³ The 2022 point in the graph is a 1-year mean and the 2021 point is a 3-year mean, so these are provisional and will change as later data are added.


Fig. 1. Annual growth of climate forcing by GHGs⁴ including the part of O₃ forcing not included in the CH₄ forcing.¹ MPTG and OTG are Montreal Protocol and Other Trace Gases.
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