National policy Statement

21.4 Statement on Earth’s Changing Climate

Effective November 10, 2021

On climate change

Earth’s climate is changing1 2. This critical issue3 poses the risk of significant environmental4, social5, and economic disruptions6 around the globe.

Multiple lines of evidence strongly support the finding that anthropogenic greenhouse gases have become the dominant driver of global climate warming observed since the mid-twentieth century7.

Moreover, the deduction that human-induced alterations to many principal components of the climate system are accelerating is supported by the preponderance of observational evidence8.

The potential consequences of climate change are great9 and the actions taken over the next decade or two10 will determine human influences on the climate for centuries to millennia.11

On climate science

As summarized in the 2018 and 2019 special reports of the Intergovernmental Panel on Climate Change (IPCC), the empirical, theoretical, and computational foundations of climate science have continually become more robust. These summaries are based upon a detailed evaluation of the evidence and quantified measures of uncertainty for each primary finding12.

On the basis of these advances, the IPCC has recently concluded13 that it is likely that human-induced warming has reached 1.0+0.2ºC since the late 19th century. During the 21st century, these estimates of human-induced warming14 have been equal to the level of observed warming to within +20%.

As recognized by prior and forthcoming Assessment Reports of the IPCC, major scientific challenges remain in our abilities to project, adapt to and mitigate anthropogenic climate change.

To better inform societal choices, the APS urges sustained research in climate science.

On climate action

The APS reaffirms its 2015 call to support actions that will reduce the emissions and ultimately the concentration, of greenhouse gases as well as increase the resilience of society to a changing climate and to support research on technologies that could reduce the climate impact of human activities.

Because physics and its techniques are fundamental elements of climate science, the APS urges physicists to expedite collaborations with colleagues across all disciplines in climate research as well as contributions to the public dialogue.

  1. In revising this statement, the evidentiary basis for statements contained in it has been drawn from reports prepared by the Intergovernmental Panel on Climate Change (IPCC – see The last major IPCC assessment was issued in 2021. The evidentiary basis for this APS Statement is drawn from both the 2021 IPCC 6th assessment and three Special Reports (SR) of the IPCC issued in 2018 and 2019. The reports are SR1.5 (“Global Warming of 1.5 ºC”), SROCC (“Special Report on the Ocean and Cryosphere in a Changing Climate”), and SRCCL (“Climate Change and Land”). Each of these reports are in turn comprised of three sections: a Summary for Policy Makers (SPM), a Technical Summary (TS) from which it is traceably derived, and the full text of the underlying report from which the TS is traceably extracted. The full references for the three Special Reports are, respectively [all URLs checked 10/8/2021]:

    • IPCC, 2018: Global Warming of 1.5°C.An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. URL:
    • IPCC, 2019: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. URL:
    • IPCC, 2019: Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems [P.R. Shukla, J. Skea, E. Calvo Buendia, V. Masson-Delmotte, H.-O. Pörtner, D. C. Roberts, P. Zhai, R. Slade, S. Connors, R. van Diemen, M. Ferrat, E. Haughey, S. Luz, S. Neogi, M. Pathak, J. Petzold, J. Portugal Pereira, P. Vyas, E. Huntley, K. Kissick, M. Belkacemi, J. Malley, (eds.)]. URL:
    • IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In press. URL:
  2. SR1.5 SPM A.1.1 thru A.1.3 – temperatures and extremes; SROCC A.1 – cryosphere; SROCC A.3 – GMSL

  3. SR1.5 SPM A.3.1 – “Impacts already observed.”; SROCC A.2 – examples include ocean heat, changes in PH, and loss of oxygen

  4. SR1.5 SPM B.3.1 thru B.3.3 on terrestrial ecosystem impacts; B.4.2 thru B.4.4 on ocean ecosystems; SROCC A.4 on cryospheric terrestrial ecosystems; SROCC A.5 and A.6, on marine and coastal ecosystems; SRCCL A.2, on desertification, land degradation, and adverse impacts on terrestrial ecosystems

  5. SR1.5 SPM B.5.1 thru B.5.4, B.5.6 – populations at risk, health, food, water stress, compound stresses; SROCC A.7 – societal impacts of shrinking cryosphere; SROCC A.8 – impacts on fisheries; SROCC A.9 – risks to coastal communities; SRCCL A.2.

  6. SR1.5 SPM B.5.5 – risks to aggregated economic growth; SROCC C1 – Economic disparities in outcomes

  7. SR11.5 SPM A.1.1, Section 1.2.1; SROCC A.2, which states “Since 1993, the rate of ocean warming has more than doubled (likely).”

  8. SR 1.5 section – it is likely that most recent warming is anthropogenic

  9. See citations above; SROCC section B on projected change– B.1 cryosphere; B.2 temps and AMOC; B.3 GMSL; B.4 land cryosphere; B.5 marine biomass; B.6 coastal ecosystems; B.7 water resources; B.8 fisheries; B.9 societal impacts of GMSL; SRCCL A.5 – climate stresses on land

  10. SR1.5 SPM Section C on scenarios, SROCC B.1 (sentence at end, B.2 (sentence at end), C.4 (opening sentences), SRCCL B.7 on positive impacts of land use scenarios; SRCCL D.1 on near-term actions; D.3 on benefits of rapid GHG emissions reductions

  11. SR1.5 SPM Section A.2

  12. Guidance Note for Lead Authors of the IPCC Fifth Assessment Report on Consistent Treatment of Uncertainties

  13. SR1.5 Technical Summary TS.1

  14. Global mean surface temperature (GMST)

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