REVIEW
CLAIM: We do not know if CO2 is the cause of global warming because global climate models and temperature records are unreliable.
In the Wall Street Journal (WSJ), opinion columnist Holman W. Jenkins claims we do not know if carbon dioxide (CO2) is the cause of global warming. Jenkins bases his November 3rd article “The Earth Is Warming, but Is CO2 the Cause?” largely on a recent report from the national statistics agency of Norway. Dozens of other outlets and blogs similarly covered that report. The claim that we do not know if CO2 is the cause of global warming is inaccurate based on a comprehensive body of scientific work, diverse methodologies and data sources, and fundamental physics, as we explain below.
A consensus of scientific evidence confirms CO2 from human emissions causes global warming
It takes more than the claims of one report to overturn decades of evidence and the international scientific consensus that CO2 causes global warming. There is a long and straightforward history of how we have reached the consensus on anthropogenic climate change, or that CO2 and other greenhouse gas emissions from humans are causing global warming.
Nearly all scientists agree humans are causing global warming through greenhouse gas emissions. Among scientists with the most climate-related expertise, the consensus reaches 100%[1]. A recent peer-reviewed scientific study analyzing thousands of other peer-reviewed scientific studies found that 99% of the scientific literature confirms human greenhouse gas emissions cause global warming[2].
Last week, the Fifth National Climate Assessment of the USA (a congressionally mandated interagency report featuring 500 expert contributors) concluded that “global warming observed over the industrial era is unequivocally caused by greenhouse gas emissions from human activities”. The report goes on to explain that CO2 is the primary greenhouse gas humans emit and the principal greenhouse gas that affects Earth’s radiative balance.
On the global stage, the reports from the United Nations Intergovernmental Panel on Climate Change (IPCC) represent humanity’s best attempts to synthesize and summarize climate change science. With each IPCC report released, thousands of experts are involved in making sure the statements are accurate and robustly supported in the scientific literature.
In the most recent IPCC report (AR6), the very first text line (line A.1.) of the “Summary for Policymakers” states “Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming”[3]. The report confirms there has been 1.1°C of global warming since the period 1850-1900 and it explicitly identifies CO2 as the leading cause (Fig. 1). As an indication of the scientific robustness of AR6, just the contribution from Working Group 1 alone was written by 234 of the world’s leading climate scientists coming from 66 countries. It included nearly 4 000 pages of research based on more than 14 000 scientific papers as supporting references and was critiqued and revised by over 1 500 expert reviewers.
Figure 1 – The contributions of different drivers to global warming from the present time period (2010-2019) relative to the time period of 1850-1900 (source). The estimates of warming (red) and cooling (blue) from radiative forcing studies (panel (c)) are based on both direct emissions into the atmosphere and their effect, if any, on other climate drivers.
Clear links between human CO2 emissions, the greenhouse effect, and global warming
We have high resolution records of CO2 increase across the globe in more recent decades, with even relatively robust proxy records before and since the start of the Industrial Revolution (which was when humans started to really harness fossil fuels like coal, petroleum, natural gas). As we burned fossil fuels throughout the industrial era, we released CO2. With current global atmospheric-CO2 concentrations now approaching 420 parts per million, human emissions have increased CO2 concentrations in the atmosphere by 50% since the year 1750 (Fig. 2).
Figure 2 – Human emissions of CO2 (grey line, right y-axis) from activities like burning fossil fuels, deforestation, and intensive agricultural land use practices represent only a small portion of total annual emissions on Earth (human and natural emissions), but they have accumulated enough year-over-year to increase atmospheric concentrations significantly (blue line, left y-axis) (source).
We have known for over a century that CO2 is a key greenhouse gas that can and is strengthening the greenhouse effect in the atmosphere. Like the other known infrared-absorbing greenhouse gases, CO2 absorbs and re-emits heat and therefore maintains a higher temperature in the atmosphere than non-greenhouse gases (Fig. 3). Since the first experiments of mid-19th century scientists, countless more studies have tested and validated the warming effect of increased atmospheric-CO2.
Figure 3 – The surface and atmosphere of the Earth absorbs solar radiation from the Sun and re-radiates it as longwave radiation. Some of the longwave radiation makes it back to space (blue shaded section). The radiation that does not, gets trapped in the atmosphere by greenhouse gases like CO2, which all have different radiative forcing strengths based on the frequencies (or wavelengths) they affect and their atmospheric concentration, among other factors. This is the greenhouse effect (red shaded section), because more longwave radiation persisting in the atmosphere produces higher air temperatures (source).
Global temperature records resulting from the efforts of thousands of data collectors, scientists, meteorologists, and other researchers across the planet, demonstrate a clear increase in temperature in recent decades that is unprecedented (Fig. 4). Various government, academic, and independent research organizations that record global temperature using varying data processing methods have reached the same results: temperature is rising. These methods and the datasets themselves are based on the gold standard of scientific publication involving peer-review critiques and re-analysis from external experts. Efforts to independently and impartially analyze temperature records, on behalf of skeptics, have confirmed their veracity repeatedly. This is how we can say the data is reliable and represents our best scientific efforts.
Figure 4 – Left: Comparison of temperature record (degrees Celsius) going back in time over 2000 years. Temperature records that are based on indirect proxy measurements are indicated by the blue trendline, while temperature records that were directly measured are indicated by the red trendline (source or source). Right: Comparison of consolidated temperature records (degrees Celsius) of multiple datasets and methodologies. The data represents temperature differences (plus or minus numbers on y-axis) between the recorded year (x-axis) and a common baseline temperature average for the period 1951-1980 (represented as 0 on the y-axis) (source).
So, we know elevated CO2 comes from human activities, which strengthens the greenhouse effect, and there has been a significant increase in global temperatures as a result. In the IPCC AR6, greenhouse gases are quantified to be (very likely) the main driver of global warming among all the different drivers (Fig. 1). Other greenhouse gases, such as methane, have been heating the atmosphere since 1850 due to their increased concentrations, but most of the temperature increase can be attributed to CO2.
Each greenhouse gas has a specific effective radiative forcing (ERF, measured in units of watts per square meter (W.m–2)) which represents the energy added (heating) or subtracted (cooling) from the Earth system due to their change in concentration and their global warming potential (GWP). Calculations of the ERF of CO2 since 1850 place it at 2.012 ± 0.241 W.m–2, while the next largest is methane (CH4) at 0.496 ± 0.099 W.m–2, followed by nitrous oxide (N2O) at 0.201 ± 0.030 W.m–2. Because these values are all positive, they represent energy added to the Earth system. They are calculated from a combined approach that uses the stratospheric-temperature-adjusted radiative forcing from radiative transfer models and adds the tropospheric adjustments derived from Earth system models[4].
The relationship between CO2 and global temperatures is so clear that we can trace recent global warming alongside recent atmospheric-CO2 increases and even predict temperatures based on the concentration of atmospheric-CO2. With each doubling of CO2, its radiative forcing increases by about 4 W.m–2, meaning there should be a logarithmic relationship between temperature and atmospheric-CO2 concentration[5]. When we plot both with CO2 on logarithmic scale and incorporate a time lag for global temperatures to respond, this is exactly what we see (Fig. 5).
Figure 5 – Left: Comparison of yearly global surface temperatures (left y-axis) with atmospheric-CO2 concentrations (grey line, right y-axis) since 1850. Yearly temperature bars are relative to average temperature from 1850-2022 with blue bars indicating a cooler than average year and red bars indicating a warmer than average year (source). Right: Global temperatures versus atmospheric-CO2 on a logarithmic scale (as described in a previous claim review on Climate Feedback).
This is why we know CO2 causes global warming. It traps infrared radiation near the Earth’s surface and is the most important driver of global warming. It may not have the highest GWP, but its concentration has increased the most in the atmosphere (in absolute terms) and it has the highest ERF. From 1990 to 2022, CO2 caused approximately 78% of the increase in global warming attributed to greenhouse gases. In summary, the greenhouse gases overall cause the most global warming of all the climate change drivers, and CO2 causes the most global warming of all the greenhouse gases (Fig. 1).
Climate change misinformation is fuelled by misconceptions, but the science is clear
Jenkins’ inaccurate claim in the WSJ refers heavily to only one source, a recent report published by Statistics Norway entitled “To what extent are temperature levels changing due to greenhouse gas emissions?”. The report authors, who do not have significant expertise in climate science, think that global climate models are probably unable to detect anthropogenic climate change. However, this climate change skepticism talking point has been addressed regularly over the years. The Statistics Norway report is problematic for numerous reasons, as demonstrated by both Dr. Rasmus Benestad, who is a senior scientist at the Norwegian Meteorological Institute with a background in physics and statistics, and by Dr. Stefan Rahmstorf, who is a physicist, oceanographer, and professor at Potsdam University.
Among the main issues with the report, the authors conflate methods of statistical models with physics-based models and misunderstand the purpose, methods, and effects of model calibration or “tuning”. Their claim that solar activity can be an explanation for global warming has long been investigated and the observations are incompatible with the hypothesis that the Sun could be the cause of global warming. In fact, the amount of energy received from the sun has decreased while the climate has warmed over the last few decades. The authors also disregard physical evidence of global-scale climate change, like sea level rise, and CO2’s greenhouse gas physical properties (proven in laboratories and by physics) by claiming local “noise” in temperature datasets makes it “impossible to determine how much of the temperature increase is due to emissions of CO2”. Local-scale datasets cannot always be expected to reflect global warming trends due to the chaotic nature of local weather. If it was only random fluctuations in temperature at the global scale, then we would expect as many decreases in temperature as increases. In reality, almost all weather stations show warming because the natural variability at the local scale, or the noise, is drowned out. The limited selection of data analyzed in the report is a poor representation of the global situation and leads to cherry picking data. This enables one to find spurious correlations or conceal significant correlations to support pre-existing biases. The authors also propose changes in Earth’s orbit as an explanation for recent climate change. While climate scientists have long been aware that changes in Earth’s orbit have caused past climate change over long periods of time, this is not something that can explain recent global warming, which is happening at a much faster rate. Both the IPCC AR6 and the NCA5 report outline how natural climate change drivers throughout the industrial era, like solar radiation and volcanic aerosols, have had negligible and regionally variable climate effects.
Jenkins also inaccurately claims that temperature records are dishonestly managed and unreliable (claims previously addressed by Climate Feedback here and here). Despite stating “the Earth is warming” in the title of his opinion piece, Jenkins contradicts himself by later writing “a future climate scandal” might in fact reveal the opposite. Claims that temperature datasets are corrupted are regularly debunked. The temperature records maintained by NOAA come from a mix of sources and have been compiled over the years to ensure full global and historical coverage. These methods are tested and verified by other bodies and researchers independent of NOAA. With each expansion of the dataset to include earlier years, more geographic locations, and higher-quality data, NOAA and other researchers using the data publish and explain the new methods and the impacts of their results. For example, researchers recently investigated new NOAA data reaching back to the year 1850 and found even more warming in the Arctic than previously estimated[6]. It is with these updates that we improve climate science and reduce the range of temperature uncertainties, which are inevitable in any effort to quantify natural phenomena at the global scale.
Conclusion
In America’s second largest print newspaper, with an online audience of millions of subscribers, Jenkins questions whether we know if CO2 drives global warming. The two authors of the Statistics Norway report claim we cannot know. However, the evidence that has been analyzed by thousands of leading scientists around the world has allowed them to conclude that, to the best of human knowledge and beyond reasonable doubt, we know that increased atmospheric-CO2 from human activity is causing global warming.
UPDATE (30 November 2023):
We updated this review to include a link to Dr. Stefan Rahmstorf’s rebuttal on RealClimate, published on 29 November 2023, and added a sentence to further explain the differences between analyzing local and global temperature datasets.
REFERENCES
- 1 – Myers et al. (2021) Consensus revisited: quantifying scientific agreement on climate change and climate expertise among Earth scientists 10 years later. Environmental Research Letters
- 2 – Lynas et al. (2021) Greater than 99% consensus on human caused climate change in the peer-reviewed scientific literature. Environmental Research Letters
- 3 – IPCC (2023) Summary for Policymakers. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
- 4 – IPCC (2021) The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
- 5 – Romps et al. (2022) Why the forcing from carbon dioxide scales as the logarithm of its concentration. Journal of Climate
- 6 – Vose et al. (2021) Implementing full spatial coverage in NOAA’s global temperature analysis. Geophysical Research Letters
