Just to simplify this discussion let’s just assume that RCP 8.5 is impossible. Does that mean CO2 levels in 2100 projected by RCP 8.5 is similarly impossible? Nope. RCP 8.5 is just one scenario. There are many other scenarios that can be imagined that can push CO2 levels at or near that projected by RCP 8.5. One attempt to quantify this was made in 2018 by William Nordhaus, a Nobel prize winner in Economics. His team applied an econometric approach to the assumptions and uncertainties of projected economic growth in the various RCP scenarios. They concluded that RCP 8.5 level emissions by 2100 had a 35% probability of occurring.
This study develops estimates of uncertainty in projections of global and regional per-capita economic growth rates through 2100, comparing estimates from expert forecasts and an econometric approach designed to analyze long-run trends and variability. Estimates from both methods indicate substantially higher uncertainty than is assumed in current studies of climate change impacts, damages, and adaptation. Results from this study suggest a greater than 35% probability that emissions concentrations will exceed those assumed in the most severe of the available climate change scenarios (RCP 8.5), illustrating particular importance for understanding extreme outcomes.
Given all this, is it reasonable to reject the “worst case scenario” levels of CO2 based on current information? I think the answer is a clear NO. Projecting carbon emissions 70 years into the future is no easy task and is full of uncertainties. The worst case scenario is probably not the most likely outcome, but it is by no means improbable and therefore must be part of the discussion.
Now you might argue that events over the past 5 years negate Nordhaus’s conclusions. Well maybe, but so much depends on whether those trends continue as expected. The nature of uncertainty is that one just can’t be sure. IMO the range of CO2 levels that we are likely to see in 2100 still includes that which would produce 103F average summer highs in Austin, which is also the conclusion of the previously linked 2020 paper.
No - it depends on those trends not being reversed. Do you think that solar panels prices are going to rise back up 10x current levels? Or that the cost of lithium battery packs will see an 8x rise in cost? That we’re going to give up on EV’s and go back to 100% ICE cars? The Nordhaus study was prepared in mid-2017 - barely five years after RCP 8.5 was first promulgated in 2012. More time has passed since the study than the amount of time he ran his analysis. And considering that his study was focused primarily on economic growth (“It will be higher than forecast”), without examining changes in carbon intensity that might have happened or were likely to happen going forward, I don’t think it’s much evidence that the changes in carbon intensity we have observed are going to be ephemeral.
So yes - I think it is reasonable to reject the “worst case scenario” levels of CO2 based on how incredibly productive investments in subsidies and research have been in driving costs of alternative energy down and adoption of alternative vehicles up. It’s Simon vs. Ehrlich, or Peak Oil, all over again - when you look at a scenario assuming that technology, policy and prices will remain similar to what they’ve been historically, you can find some very dire possibilities. But technology, policy, and markets are pretty good at finding solutions that will avert catastrophe - not always, but it’s hardly surprising or unusual when they do.
We’ve made a lot of investments in climate solutions in the last decade or so - investments that are only recently beginning to bear fruit. We paid a lot of money in subsidies, direct research grants, higher energy prices/feed-in tariffs, and import policy to drive down the cost of solar panels, wind generation, and electric vehicles - so the energy and transportation markets don’t look like they did in 2017. It would be foolish to assume that the path forward from here will look anything like the presumed “no action on climate” projections that underlay the worst case scenarios back in 2012.
Probably not. On the other hand, I do think it possible that India, China, and the young Asian Tiger economies might increase coal use to push economic growth, particularly if they are facing economic difficulties. I do think it possible that climate change and land use will create substantial losses of the Amazon forests, a major carbon sink. I do think it possible that climate change will cause major human migrations that could disrupt the international collaborations required to reduce carbon emissions. I do think it possible that republicans might control the government and delay the world’s largest economy from getting to carbon zero by a decade or two.
Will the western economies really spend hundreds of billions of dollars to transition the 3rd world to sustainable energy? I don’t know, but it doesn’t look promising. China may go 100% EV soon, but it is all for nought if the electricity for these cars is coming from coal. As DrBob often posts, China continues to build coal plants at a rapid pace. Will they be used or are they just to prop up the local economy? Who knows?
The UN estimates that since 2010 the world loses a net of 4.7million hectares of forests per year. Do you think that is going to be reversed any time soon? The wildfires currently burning through millions of acres of Canadian forest are expected to increase in frequency with climate change.
And then there is the possibility of the stuff not modeled. For example, the Arctic is becoming ice free, exposing the previous frozen soil to melting. This permafrost is estimated to contain billions of tons of carbon. There is a some concern that warming will release much of this carbon into the atmosphere, none of which is incorporated in current climate modeling.
So there are lots of trends, some negative some positive. The only way to know for sure what side is prevailing is to look at estimates of carbon emissions (not projections, but actual measurements) and atmospheric CO2 levels.
The issue isn’t whether RCP 8.5 is plausible. The issue is whether global carbon emissions will decline by mid-century and then carbon-neutrality by the end of the century. The longer it takes for those events to occur, the more likely the worse case scenario becomes. How likely is it that most nations will meet their climate pledges? The historical track record is not promising.
But that doesn’t affect the analysis here. Because all of the negative trends were already included in RCP 8.5. It was the worst case scenario - the scenario where we did nothing to fight climate change.
But we have made material steps that reduce climate emissions. Fracking has allowed a massive transition away from coal to natural gas. The cost of solar generation and wind generation has collapsed due to technological advances and trade policy. Technology and efficiency have also collapsed the cost of lithium batteries, leading to widespread adoption of EV’s. Etc.
Yes, there’s lots of trends that are negative. Many will continue to be negative. That’s why we’re heading to a +2.5 - 3.0 C world, which is not great. But those are part of RCP 8.5. It’s the scenario where everything goes wrong, where there are no positive changes. The massive positive changes that have happened have moved us out of an RCP 8.5 scenario - which is why we’re no longer realistically moving to a +5.0 - 6.0 C world. We haven’t done enough to avoid massive problems - but we have done enough to avoid armageddon.
Is it feasible that we will reverse all of those positive changes? That we’ll shift back from fracking to coal, that solar and wind and battery prices will go back to being 10x current levels, that we’ll reverse EV adoption and go back to 100% ICE? I don’t think it’s even possible - but it’s certainly a contingency so remote we would never seriously consider it as part of our policy making…
The primary reasons RCP 8.5 has such high CO2 emissions is that it gives a high estimate of fossil fuel use and assumes a relatively high global population. Its projections of deforestation and changes in land use are not that different from that incorporated in the other RCPs. So things could easily be worse than RCP 8.5 if the decline of tropical forests is worse than expected or coal use extends longer than expected.
The RCPs are all built on economic assumptions that the Nordhaus paper indicates are sufficiently uncertain to allow for very large outcome differences, including a 35% likelihood of high CO2 atmospheric levels by end of century. Even if you reduce that probability by an order of magnitude, a 3% probability of an uninhabitable Austin TX is uncomfortably high.
The RCPs do not take into account a number of potential positive feedbacks. That’s not because of a determination that these are unlikely. It is because there is simply not enough information to do modeling. It is likely that one or more of these are going to be significant, but we just don’t know. Nevertheless, any rational risk assessment has to take these into consideration.
Finally, only the most stringent scenario, RCP 2.6, projects global carbon neutrality by 2100. That means for all the other scenarios, temperatures are going to continue to increase after 2100. So maybe Austin TX “only” averages 99F in the summer of 2100, but it might get to 103F by 2150.
That may be your legal opinion, but it is not supported by science. There is simply too much uncertainty. For example, polar ice melt appears to be slowing ocean currents. This in turn could have substantial negative impacts on ocean biota and reduce the uptake and storage of CO2 by the seas. We just don’t know but there is a lot of suggestive evidence that it will be impactful. The ocean is a pretty big thing for us to be taking risks with. https://www.nature.com/articles/s41558-022-01555-7
All very cogent arguments regarding the uncertainty of climate predictions - but again, none of that uncertainty is really new. RCP 8.5 was always only one scenario, subject to a lot of uncertainty, even when it was first formulated in 2012. We now know, however, that RCP 8.5 is inaccurate in key ways - because we have made significant changes in human-caused emissions that on a going-forward basis will move us away from the RCP 8.5 emissions scenario. While there are (and always will be) error bars around the emissions scenario, it’s no longer sensible to use RCP 8.5 as your starting emissions scenario instead of something like RCP 6.0.
It kind of got lost in the RCP discussions, but I think your assessment that Austin would be uninhabitable is wrong. Austin’s heat and humidity are negatively correlated. Their hottest month (August) is their driest month. Humidity peaks in the morning, when it is cooler, and reaches a daytime low right around the temperature peak in the afternoon. So when temperatures are hitting those 100 degree marks - high summer afternoons - humidity is going to be around 40-50%, not 70%. Giving you a wet bulb temp of mid-80’s:
These are just averages, of course - there will undoubtedly be unusually hot and humid days from time to time. But Austin won’t be uninhabitable if the temps go up even by several degrees, because the average afternoon humidity just isn’t high enough.