Arguments
The new IPCC Report includes – get this, good news
Posted on 13 August 2021 by dana1981
This is a re-post from Yale Climate Connections
As the Intergovernmental Panel on Climate Change (IPCC) released its Sixth Assessment Report, summarized nicely on these pages by Bob Henson, much of the associated media coverage carried a tone of inevitable doom.
While United Nations Secretary-General António Guterres rightly called the report a “code red for humanity,” tucked into it are details illustrating that if – BIG IF – top-emitting countries respond to the IPCC’s alarm bells with aggressive efforts to curb carbon pollution, the worst climate outcomes remain avoidable.
The IPCC’s future climate scenarios
In the Marvel film Avengers: Infinity War, the Dr. Strange character goes forward in time to view 14,000,605 alternate futures to see all the possible outcomes of the Avengers’ coming conflict. Lacking the fictional Time Stone used in this gambit, climate scientists instead ran hundreds of simulations of several different future carbon emissions scenarios using a variety of climate models. Like Dr. Strange, climate scientists’ goal is to determine the range of possible outcomes given different actions taken by the protagonists: in this case, various measures to decarbonize the global economy.
The scenarios considered by IPCC are called Shared Socioeconomic Pathways (SSPs). The best-case climate scenario, called SSP1, involves a global shift toward sustainable management of global resources and reduced inequity. The next scenario, SSP2, is more of a business-as-usual path with slow and uneven progress toward sustainable development goals and persisting income inequality and environmental degradation. SSP3 envisions insurgent nationalism around the world with countries focusing on their short-term domestic best interests, resulting in persistent and worsening inequality and environmental degradation. Two more scenarios, SSP4 and SSP5, consider even greater inequalities and fossil fuel extraction, but seem at odds with an international community that has agreed overwhelmingly to aim for the Paris climate targets.
The latest IPCC report’s model runs simulated two SSP1 scenarios that would achieve the Paris targets of limiting global warming to 1.5 and 2°C (2.7 and 3.6°F); one SSP2 scenario in which temperatures approach 3°C (5.4°F) in the year 2100; an SSP3 scenario with about 4°C (7.2°F) global warming by the end of the century; and one SSP5 ‘burn all the fossil fuels possible’ scenario resulting in close to 5°C (9°F), again by 2100.
Projected global average surface temperature change in each of the five SSP scenarios. (Source: IPCC Sixth Assessment Report)
The report’s SSP3-7.0 pathway (the latter number represents the eventual global energy imbalance caused by the increased greenhouse effect, in watts per square meter), is considered by many experts to be a realistic worst-case scenario, with global carbon emissions continuing to rise every year throughout the 21st century. Such an outcome would represent a complete failure of international climate negotiations and policies and would likely result in catastrophic consequences, including widespread species extinctions, food and water shortages, and disastrous extreme weather events.
Scenario SSP2-4.5 is more consistent with government climate policies that are currently in place. It envisions global carbon emissions increasing another 10% over the next decade before reaching a plateau that’s maintained until carbon pollution slowly begins to decline starting in the 2050s. Global carbon emissions approach but do not reach zero by the end of the century. Even in this unambitious scenario, the very worst climate change impacts might be averted, although the resulting climate impacts would be severe.
Most encouragingly, the report’s two SSP1 scenarios illustrate that the Paris targets remain within reach. To stay below the main Paris target of 2°C (3.6°F) warming, global carbon emissions in SSP1-2.6 plateau essentially immediately and begin to decline after 2025 at a modest rate of about 2% per year for the first decade, then accelerating to around 3% per year the next decade, and continuing along a path of consistent year-to-year carbon pollution cuts before reaching zero around 2075. The IPCC concluded that once global carbon emissions reach zero, temperatures will stop rising. Toward the end of the century, emissions in SSP1-2.6 move into negative territory as the IPCC envisions that efforts to remove carbon from the atmosphere via natural and technological methods (like sequestering carbon in agricultural soils and scrubbing it from the atmosphere through direct air capture) outpace overall fossil fuel emissions.
Meeting the aspirational Paris goal of limiting global warming to 1.5°C (2.7°F) in SSP1-1.9 would be extremely challenging, given that global temperatures are expected to breach this level within about a decade. This scenario similarly envisions that global carbon emissions peak immediately and that they decline much faster than in SSP1-2.6, at a rate of about 6% per year from 2025 to 2035 and 9% per year over the following decade, reaching net zero by around the year 2055 and becoming net negative afterwards.
Global carbon dioxide emissions (in billions of tons per year) from 2015 to 2100 in each of the five SSP scenarios. (Source: IPCC Sixth Assessment Report)
For perspective, global carbon emissions fell by about 6-7% in 2020 as a result of restrictions associated with the COVID-19 pandemic and are expected to rebound by a similar amount in 2021. As IPCC report contributor Zeke Hausfather noted, this scenario also relies on large-scale carbon sequestration technologies that currently do not exist, without which global emissions would have to reach zero a decade sooner.
More warming means more risk
The new IPCC report details that, depending on the region, climate change has already worsened extreme heat, drought, fires, floods, and hurricanes, and those will only become more damaging and destructive as temperatures continue to rise. The IPCC’s 2018 “1.5°C Report” had entailed the differences in climate consequences in a 2°C vs. 1.5°C world, as summarized at this site by Bruce Lieberman.
Consider that in the current climate of just over 1°C (2°F) warmer than pre-industrial temperatures, 40 countries this summer alone have experienced extreme flooding, including more than a year’s worth of rain falling within 24 hours in Zhengzhou, China. Many regions have also experienced extreme heat, including the deadly Pacific Northwest heatwave and dangerously hot conditions during the Olympics in Tokyo. Siberia, Greece, Italy, and the US west coast are experiencing explosive wildfires, including the “truly frightening fire behavior” of the Dixie fire, which broke the record as the largest single wildfire on record in California. The IPCC report warned of “compound events” like heat exacerbating drought, which in turn fuels more dangerous wildfires, as is happening in California.
Western North America (WNA) and the Mediterranean (MED) regions are those for which climate scientists have the greatest confidence that human-caused global warming is exacerbating drought by drying out the soil. (Source: IPCC Sixth Assessment Report)
The southwestern United States and Mediterranean are also among the regions for which climate scientists have the greatest confidence that climate change will continue to increase drought risk and severity. (Source: IPCC Sixth Assessment Report)
The IPCC report notes that the low-emissions SSP1 scenarios “would lead to substantially smaller changes” in these sorts of climate impact drivers than the higher-emissions scenarios. It also points out that with the world currently at around 1°C of warming, the intensity of extreme weather will be twice as bad compared to today’s conditions if temperatures reach 2°C (1°C hotter than today) than if the warming is limited to 1.5°C (0.5°C hotter than today), and quadruple as bad if global warming reaches 3°C (2°C hotter than today). For example, what was an extreme once-in-50-years heat wave in the late-1800s now occurs once per decade, which would rise to almost twice per decade at 1.5°C, and nearly three times per decade at 2°C global warming.
The increasing frequency and intensity of what used to be 1-in-50-year extreme heat as global temperatures rise. (Source: IPCC Sixth Assessment Report)
Climate’s fate has yet to be written
At the same time, there is no tipping point temperature at which it becomes “too late” to curb climate change and its damaging consequences. Every additional bit of global warming above current temperatures will result in increased risks of worsening extreme weather of the sorts currently being experienced around the world. Achieving the aspirational 1.5°C Paris target may be politically infeasible, but most countries (137 total) have either committed to or are in the process of setting a target for net zero emissions by 2050 (including the United States) or 2060 (including China).
That makes the SSP1 scenarios and limiting global warming to less than 2°C a distinct possibility, depending on how successful countries are at following through with decarbonization plans over the coming three decades. And with its proposed infrastructure bipartisan and budget reconciliation legislative plans – for which final enactment of each remains another big IF – the United States could soon implement some of the bold investments and policies necessary to set the world’s second-largest carbon polluter on a track consistent with the Paris targets.
As Texas Tech climate scientist Katharine Hayhoe put it,
Again and again, assessment after assessment, the IPCC has already made it clear. Climate change puts at risk every aspect of human life as we know it … We are already starting to experience those risks today; but we know what we need to do to avoid the worst future impacts. The difference between a fossil fuel versus a clean energy future is nothing less than the future of civilization as we know it.
Back to the Avengers: They had only one chance in 14 million to save the day, and they succeeded. Time is running short, but policymakers’ odds of meeting the Paris targets remain much better than that. There are no physical constraints playing the role of Thanos in our story; only political barriers stand between humanity and a prosperous clean energy future, although those can sometimes be the most difficult types of barriers to overcome.
This makes no sense to me. We can still avoid the 2C limit by reducing CO2 emissions by just 2-3% per year? And aren't there positive feedback mechanisms that will cause the world to keep warming after emissions hit zero (e.g., artcic methane release)?
Current emissions are 38 billion metric tons per year. If we integrate (38 * 10^9)*(0.97^x) from x = 0 to x = 55 we get 1 trillion tons of CO2 that will be emitted by 2075, and I haven't seen a carbon budget for two degrees that's anywhere near that big. So a 3% reduction per year wouldn't be nearly enough.
This source here says we have 469 billion metric tons left, and that was written in 2013, meaning that now we only have about 200 billion metric tons left, which at current rates means we will blow the two-degree target in only five years.
www.climatecentral.org/news/ipcc-climate-change-report-contains-grave-carbon-budget-message-16569
Perhaps better data since then might give us a better budget, but could it change by that much?
anticorncob6. My understanding from what I've read is if we stop emissions by for example 2050 temperatures will stop increasing quite soon after that, and the the arctic will stop melting fairly quickly, and acting as a positive feedback to warming, provided it hasn't crossed a tipping point where melting has become self reinforcing. This scenario makes sense to me.
It's not certain just when the arctic would cross a tipping point and it may have already. From memory the range is 1-3 degrees c. I could be wrong on all this and maybe someone has better information.
anticorncob6 @1,
The CO2 budgets quickly become very complicated and comparing them takes a little spade-work. Here is my simplistic take on it.
That 2013 Carbon Budget you link to is quite a generous one, even though it is for +2ºC AGW. Its 1,000Gt(C) emissions budget or 3,664Gt(CO2) with an Airborne Fraction of 45% would yield [1,000 x 0.45 / 2.13 + 275ppm=] 486ppm atmospheric CO2 by 2090 (followed by negative emissions). Other Carbon Budgets, for instance the IPCC SR1.5 budget from 2018 set a budget at 432ppm for a 66% chance of avoiding +1.5ºC AGW, this with large negative emissions to follow the reaching of zero. (My assessment here using the simplistic Af=45%.) The AR6 SSP1-2.6 with its 2% annual reductions for a +2ºC AGW, again followed by negative emissions post 2075, I'd assess at something like 285Gt(C) post-2020 so 474ppm, not greatly less than that 2013 Budget you linked to @1. Mind the real wake-up numbers come from AR5 which put the 66% +1.5ºC AGW at 417ppm.
You mention the "positive feedbacks" and perhaps nigelj @2 should have added that land ice will continue melting away unless global temperatures are reduced, the worry being that Greenland will melt down (taking millennia) with warming somewhere between +1.0ºC & +2.0ºC AGW and with nothing to stop it once its summit drops down to warmer altitudes. And the stability of the West Antarctic ice is potentially even more sensitive to warming.
Specific to being "positive feedbacks" (which melted ice fields are not unless they entirely disappear & so reduce albedo), the melting tundra is also a process which will continue for centuries without a return to a chillier climate. The size of such the feedback from melting tundra will depend on how hot we make it.
Keeping the ice sheets intact and the tundra frozen is one of the more obvious reasons why limiting AGW to +1.5ºC is a sensible policy.
Ok, so there's still something I don't get (maybe it's a dumb question and feel free to ignore it).
So at 280 ppm CO2 (and whatever other GHGs at the time), the earth was at a given avg temp. Climbing up to 410 ppm CO2 (plus other GHGs), Anthro forcing brought the temp up at least 1.1 degrees C.
Over 150 years (using 1850 as baseline).
So, as we go back down the CO2 curve over the next 80 years, it seems hard to believe we won't increase the temperature by more than .5 C.
And this assumes the earth system continues to absorb over half of human emissions.
Odd they suggest there is "no tipping points temperature" since there is evidence we might have crossed it already:
https://geography.exeter.ac.uk/staff/index.php?web_id=Timothy_Lenton
ilfatk2 @4,
I assume you are asking how additional warming will be constrained to keep us under +1.5ºC if we are already at +1.1ºC. Thus what you describe being "as we go back down the CO2 curve over the next 80 years" concerns our reduction in CO2 emissions in coming decades.
The processes that will determine whether we will keep warming under +1.5ºC are detailed and complex but I am assuming you are happy with the idea that when we do at last reduce CO2 enissions to zero, the global temperature rise will effectively stop.
(If you are not entirely clear on the reason for this 'stopping', it is because the "earth system" is not actually absorbing half our emissions. It is absorbing annually a level of CO2 that happens to be equal to roughly half our annual emissions. In rough numbers, we emit into the atmosphere 10Gt(C) per year while the planet sucks 5Gt(C) out of the atmosphere in the year. Very little of this 5Gt(C) 'in the year' is due to our emissions 'in the year'. If we didn't emit any CO2 next year our rough absorption number would only drop by something like 0.3Gt(C) because what is being absorbed is the accumulation of our CO2 emissions thrugh the years.)
So if AGW effectively stops when we achieve zero emissions and 2050, in three decades time, is often given as a lacklustre target date consider the rate of AGW is today running at something like +0.25ºC per decade. So at that rate we would see +0.75ºC additional warming by 2050.
But we hopefully will not be mad enough to run up to 2050 without large cuts running through those three decades 2021-50. So that +0.25ºC/ per decade will drop with successive decades, perhaps halving that post-2021 warming, say +0.20ºC+0.15ºC+0.5ºC=+0.40ºC, and allowing AGW to be restricted to +1.5ºC.
I hope that makes sense of the basics of the situation.
It will require deep cuts and even then note the negative emissions factored-in post-2050 to achieve +1.5ºC.
Sadly I am not entirely convinced that politicians (& I speak here for UK which is where I am) are truly understanding of the deep deep do-do we are facing and why deep deep cuts in emissions are essential and essential quickly. Instead the process seems to be drowning in greenwash.
I note your comment on tipping points and the link to the Lenton biog. My understanding is that the likes of Lenton are calling for the +1.5ºC limit to be taken seriously (as per Lenton et al (2020) 'Climate tipping points — too risky to bet against'). I am not aware of stronger scientific messaging although I would say that +1.5ºC is not without risks and is a level of warming that should be avoided rather than be seen as some target.
thanks very much
To make the point maybe a bit more succinct:
What the terresrial bisophere and the ocean take up each year is driven by atmospheric concentrations of CO2, not annual man-made CO2 emissions. While the latter change dramatically in the SSP1 scenarios, the former do not. Atmospheric CO2 levels drop only slowly as the new equilibria get established over different timescales.
I consider it extremely dangerous or even fatal for life on earth to rely solely on the future, hypothetical CO² emissions of mankind, which also depends primarily on the psychological readiness of capital-driven, ignorant fossil fuel junkies . The rise in temperature that has taken place to this day has put a lot of psychological, physical, biological and ecological (just to name the most important) evolutionary pressure and feedback into operation, which in their interaction and mutual reinforcement cannot be calculated or estimated by (climate) science at all. The 30 years to 2050 can be damn long if you have one leg in hell and the other on the fire hose.