Climate models of future warming (Don Bogard) USofA

CLIMATE MODELS OF FUTURE WARMING: UNKNOWNS & UNCERTAINTES Donald Bogard, December, 2018 Background. United Nations (UN) and US Reports on Climate Change (particularly the two recent US National Climate Assessment) focus on the nature and extent of climate change, what drives climate change, and what might be the consequences if future warming is not curtailed. These reports often paint a dire future, which has been given wide dissemination by the press. International studies into these questions are sponsored under the UN and its study group, the International Panel on Climate Change (IPCC). Studies and reports of the IPCC over many years have defined a range of probabilities for future temperature. The U.S. National Climate Assessment Program has operated in parallel, but in similar ways to the UN-IPCC studies, and with an emphasis on the U.S. They issued a report on physical climate change in 2017 (https://science2017.globalchange.gov/) and recently a 2018 report on what might be effects of predicted climate changes across a broad spectrum of human activity and infrastructure (https://nca2018.globalchange.gov/).

Is future global temperature rise, as modeled and utilized in these reports by the UN-IPCC and US Climate Assessment, the only interpretation and how certain is it? Future temperature predictions depend on multiple factors, some of which are either uncertain or unknowable. My purpose here is to give an overview of the results of temperature predictions from climate models as developed by the IPCC and to illustrate that even the IPCC recognizes a wide range of future temperature possibilities. What negative consequences of future warming might result (e.g. those emphasized in the recent US Climate Assessment report) obviously depend on what the unknown future temperature may be. Some climate professionals contend that the future temperature emphasized in the US Climate Assessment report lies at the extreme high end of the range of possibilities determined by the IPCC, and thus it greatly overemphasizes likely future outcomes.

Whether you accept the predictions of these climate reports or not, it is informative to know some of the assumptions and uncertainties involved in their derivation.

Increasing CO2 and RCP options.

Many factors are capable, in principle, of altering global temperature. One obvious possibility is a change in energy output of the Sun, which is the source of essentially all incoming energy to planet Earth. The expressed UN-IPCC major concern, however, is with increasing concentration of atmospheric greenhouse gases, especially CO2. Atmospheric CO2 has risen by about 40% since the late 19th century, and much of that increase can be attributed to human activities, especially burning of fossil fuels. Obviously, in order to model future warming based on increasing CO2, assumptions about future CO2 growth must be made. The UN-IPCC does this by using Relative Concentration Pathways (RCP). Four RCP values have been defined, 8.5, 6.0, 4.5, and 2.6, where the numbers represent Earth’s energy imbalance in watts per square-meter of surface produced by increasing concentrations of greenhouse gases. This energy imbalance is referred to as radiative forcing, and these RCP values are associated with specific increased amounts over time of atmospheric CO2 (also CH4, N2O, etc.). They can be indirectly associated with an amount of global warming, as discussed below. Here we pass over the considerable details involved in making these associations among energy imbalance, atmospheric CO2 increase, and amount of temperature rise and examine only the final IPCC output.

Figure 1 (next page, from IPCC fifth report, 2013) shows modeled temperature increases up to year 2200 for three of these RCP models. (Gray shaded regions show temperature uncertainties associated with each RCP model.) Each RCP value assumes some specific manner in which atmospheric CO2 increases over time and global temperature responds. RCP 8.5 assumes CO2 continues to increase, and thus predicts continually increasing temperature to year 2200. RCP 6.0 assumes atmospheric CO2 eventually stabilizes and the

temperature increase slows after year 2100, as the atmospheric CO2 equilibrates with its exchange reservoirs (oceans, plants, soil, etc.) and effects of the prior warming equilibrate throughout the system. RCP 4.5 assumes CO2 stabilizes within a few decades and the temperature increase significantly slows after ~2050. Although Figure 1 shows the global temperature path under each assumed RCP scenario, it does not show the assumed future growth of atmospheric CO2 under each RCP. The red horizontal line marks the 2 degC of global warming (relative to late 19th century) that the UN-IPCC has established as a desired upper limit. Note that these three RCP pathways up to that point are similar and there is no difference in temperature paths of RCP 6.0 and 4.5.

Figure 2 (right, from reference 2) shows these same three hypothetical RCP pathways with the Y-axis being the modeled net yearly increase in atmospheric CO2 in Gt (gigatons). (Zero on the Y-axis represents atmospheric CO2 in the 19th century.) Also given is RCP 2.6.

This graph more clearly shows how future atmospheric CO2 concentrations are defined to evolve within these pathways.

Obviously, these defined increases in CO2 are arbitrary and the exact path future atmospheric CO2 may follow cannot be determined by climate models. Under RCP 4.5 CO2 begins to decrease about 2040 down to levels present around the middle of the 20th century. Under RCP 2.6 CO2 begins to decrease about 2020 and later in the century it falls below the current atmospheric CO2 level, likely requiring CO2 removal from the atmosphere.

The approximate highest atmospheric CO2 concentrations in ppm for each RCP are given in the upper left of the figure. (Currently Earth’s atmosphere contains ~3200 Gt (giga-tons) of CO2, or ~400 ppm.)

An important element of Figure 2 is the many colored lines associated with each RCP curve. These represent the individual paths of many climate models run under that specific RCP scenario. They differ because of different assumptions about input parameters. Overall, these model runs define the heavy lines that are the RCP pathways. The variability in individual models produces the range in highest atmospheric CO2 concentrations given in the upper left and the temperature range on the right. Importantly, not only is the specific RCP path future atmospheric CO2 might follow unknown, but the CO2 and temperature evolution associated with that RCP are also poorly defined.

Why Are Modeled Future Temperatures Uncertain?

If future increases in atmospheric CO2 and other greenhouse gases are assumed within a specific RCP, then why do individual climate model predictions of

future temperature for a specific RCP vary so widely? Essentially all climate models assume little influence from solar variations. The answer lies in assumptions made about various so-called feedback factors that influence temperature. Separate from direct energy forcing functions (e.g., the Sun), temperature changes arising from increased greenhouse gases can be divided into two parts. One is the direct warming produced by the increased CO2 (CH4, N2O etc.) itself (which I explain in my science summary “How CO2 Causes Warming”). The direct global temperature increase when atmospheric CO2 is doubled (e.g., from 300 ppm to 600 ppm) is reasonably well estimated at ~1.1 deg-C. Feedback factors are secondary and can be activated when increased CO2 (CH4, etc.) produces a global temperature increase. Feedback factors are more uncertain, can be either positive (causing warming) or negative (causing cooling), and it is possible that not all relevant feedback factors are identified. One example of a complex feedback factor occurs when a global temperature increase produces greater evaporation of water into the atmosphere. That added water vapor produces additional warming because water vapor is the most effective greenhouse gas. However, part of that water forms more clouds. Some clouds increase Earth’s albedo and reflect away more solar radiation, causing cooling. (Earth’s overall albedo from several causes is ~30%.) Earth’s surface and atmosphere are continuously shedding gained solar energy by radiating infra-red (IR) radiation toward space. But some types of clouds reflect that IR energy back to Earth, slowing energy loss, and thus produce cooling. The overall effects from increased water vapor and increased clouds is perhaps the most uncertain feedback factor.

Aerosols are also a complex factor in determining temperature (mostly cooling).

Figure 3 (right) shows various feedback factors assumed by the UN-IPCC in modeling possible future temperatures.

These are an agreed-upon set of factors produced from many individual investigators estimates, which sometimes differ considerably from these IPCC values.

Figure 4 (below) compares some feedback factors presented by several individual investigations. Note the variations.

CO2 climate sensitivity. To predict a future temperature produced by increasing greenhouse gases requires an adopted value for all warming from these gases, both direct and the net effect of feedback factors. A common way to express this is called the CO2 climate sensitivity, which gives an amount of expected warming whenever the atmospheric CO2 concentration is doubled.

Over the years, estimates of this CO2 sensitivity have been declining, as indicated by Figure 5 (right, reference 5). The red and blue data represent transient climate response (TCR) and equilibrium climate sensitivity (ECS) and represent two different ways to increase atmospheric CO2, gradually over time or all at once and wait for temperature to come to equilibrium. The CO2 climate sensitivities reported by both methods have been decreasing. Many recent determinations of this sensitivity have been around 2 deg-C for CO2 doubling, and some investigations argue for sensitivities between 2 and 1 deg-C per CO2 doubling (where a sensitivity of 1 would imply that negative feedbacks offset positive ones).

The IPCC recognizes, because of several uncertainties in input parameters, that there exists a range of possible sensitivities, and their reports reflect this ambiguity in their wording. In the fifth report (2013) the IPCC expresses this range of warming uncertainty for CO2 doubling as follows. "There is high confidence that ECS is extremely unlikely less than 1°C and medium confidence that the ECS is likely between 1.5°C and 4.5°C and very unlikely greater than 6°C”.

The fourth IPCC report (2007) concluded this sensitivity “was likely to be in the range 2 to 4.5 °C with a best estimate of about 3 °C, and is very unlikely to be less than 1.5 °C. Values substantially higher than 4.5 °C cannot be excluded, but agreement of models with observations is not as good for those values. The third IPCC report (2001) said the sensitivity was "likely to be in the range of 1.5 to 4.5 °C". Note from Fig. 2 that RCP 6.0 predicts a temperature increase by 2100 in the range 2.0-3.7 deg-C and a maximum atmospheric CO2 concentration between 580 and 720 ppm. The IPCC has given quite low probability for the RCP 8.5 scenario, where in 2100 temperature would approach 3.2-5.4 deg-C and CO2 would exceed 1,000 ppm (Fig. 2).

Many investigations have utilized observed historical or proxy data to estimate CO2 climate sensitivity.

These results disperse widely and tend to have large uncertainties (reference 6). There also has been debate over whether TCR or ECS climate sensitivities (Fig. 5) are preferred. Still other investigations argue that the IPCC-adopted sensitivities ignore likely large temperature forcing from the variable Sun. They often point out the correlation between solar magnetic and sunspot activity with lower global temperature during and after the Little Ice Age a few hundred years ago. Others argue that significant variations in global temperature over the past ~150 years requires that a significant portion (possibly half) of observed warming is driven by natural processes, thus requiring the CO2 sensitivity to be appreciably lower than estimated by the IPCC. These many facets of the issue are too diverse and complex to discuss here, but exist nevertheless.

The take-away point is that the UN-IPCC estimates of the amount of expected future warming based on increasing atmospheric CO2 has a considerable uncertainty range. Non-IPCC investigations often argue that the CO2 sensitivity is even lower than suggested by the IPCC. Additional relevant observations. None of these investigations know what the actual future increase in atmospheric CO2 will prove to be. Greenhouse gas emissions come from a variety of sources. Burning of fossil fuels is the major source, but land use practices (agriculture, husbandry, forest burning, etc.) is also a significant source. Some argue that over time fossil fuels have become more expensive to extract and this will encourage greater use of other energy sources. Others argue that recoverable fossil fuel deposits are limited and may limit future atmosphere CO2 to ~600 ppm. Currently it is unknown to what extent

renewable energy sources (wind and solar) and nuclear may replace the use of fossil fuels. Current growth in world CO2 emissions is highly concentrated in developing countries; little to no growth occurs in the U.S. and Europe. Most of these developing countries give greater priority to increasing their living standard than reducing greenhouse emissions. Therefore, which RCP scenario might more closely align with the actual future increase in greenhouse gases is impossible to predict. Even if it were possible to predict a future RCP scenario, the amount of warming that might produce is quite uncertain.

I return to my comment in the opening paragraph that the recent National Climate Assessment, paints a dire future and point out the following distortions in this report. Most of the future predictions are based on the highest RCP scenario, RCP 8.5, which the IPCC considers low probability, and seldom consider effects of lower and more probable RCP scenarios. That may have been intentional, as one source (reference 7) suggested the Obama administration told the authors of the latest National Climate Assessment (NCA) to emphasize “worst-case� global warming predictions. The economic damage predicted from this exaggerated expected warming was also significantly exaggerated and is higher than the worst-case scenario predicted by the UN-IPCC. In addition to assuming a RCP scenario of 8.5, the National Climate Assessment makes other unlikely assumptions. One is rapid population growth to 12 billion people. Studies have shown that as the living standard of a nation increases, its population growth slows, even stops, and in some cases becomes negative. The assessment also assumes an unusually low rate of technology development, slow GDP growth, and a massive increase in world poverty. All these seem improbable as long as energy is affordable and available. The assessment also assumes nearly impossible levels of coal consumption, fails to take into account the massive increase in natural gas production from the shale revolution, and ignores technological innovations that continue to occur in nuclear and renewable technologies. The National Climate Assessment report also assumes that several major weather events (floods, storms, fires, etc.) over the past several years are the cause of increasing temperature and will continue to get worse. Some investigators have presented data arguing this is unlikely. Further, this report exaggerates future costs of global warming, while ignoring increased costs associated with any rapid and drastic changes in energy sources and land use.

I find it interesting that as claims of future catastrophe from global warming become more shrill and desperate (as with the US National Climate Assessment, Part 2) attempts to decrease greenhouse emissions through decreases in fossil fuel burning are discovering issue after issue that must be overcome. It is one thing to argue that greenhouse gas emission must decrease and stop, but it is another to actually find ways to accomplish that. Modern society is built on energy, and world populations desire to increase both energy use and their living standard. What studies such as the National Climate Assessment should do is to investigate practical and affordable ways to utilize energy sources that do not release greenhouse gases and ways to utilize the land efficiently. IF these future warming predictions have any reality, an increase in global temperature of 2 deg-C, relative to 19th century temperature, cannot be avoided by any action.

Reference 2. Fuss et al., Nature Climate Change, http://www.globalcarbonproject.org/global/pdf/Fuss_2014_Betting%20on%20negative%20emissions.NatureCC.pdf

Reference 5. Scafetta et al., http://www.iieta.org/sites/default/files/Journals/IJHT/35.Sp01_03.pdf Reference 6. https://en.wikipedia.org/wiki/Climate_sensitivity Reference 7. https://dailycaller.com/2018/11/28/epa-andrew-wheeler-obama-climate-report/