Is Global Warming "Real, Man-made and Dangerous?"
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| GMST in Various Datasets |
Since 2013 or so, it's been common to refer to global warming over the last 100 years or so as "real," "man-made" and "dangerous."[1]. To my knowledge, these terms were first used in political discussion, but I believe they can be useful categories for discussing whether it's happening (real), whether it's something we can correct (man-made) and whether it's a problem that needs correcting (dangerous). These three categories of claims do a good job of summarizing how a case can be built that we have a problem that we both can fix and need to fix. These categories are valuable because 1) we can only potentially have a climate warming problem if global warming is real. If it's 1) real and it's 2) man-made, then we have the capacity to fix it by changing what we're doing. If, however, it's mostly natural, then potentially there's nothing we could do to fix the problem, and that's really bad news. And if it's both 1) real and 2) man-made, there's no need to fix the problem unless the problem is also 3) dangerous. Even better, these three valuable categories can be tested against empirical evidence. If these claims are correct, then there's a real and dangerous problem that we need to fix, and we can fix it if we change what we're doing to cause the problem.
So let's deal with each in turn. The argument is complex, and I can't be complete in a single blog post. I included many references below so you can check up on what I say here. And when details for part of the argument are contained in other posts, I link to those posts without necessarily repeating those details here to save space. As I deal with many of these issues in future posts, I plan to add more links with more details than I've outlined here as well. For the record, I consider this a compelling but incomplete case. Much more can be said, and much more will be said here as I'm able to give time so some of the complexities behind what I say below.
Global Warming is Real
The globe is warming. GMST is now about 1.2 C warmer than the 1850-1900 mean, and the rate of change has accelerated to > 0.2 C/decad. This is true in all GMST datasets, which show similar warming since 1850 (or 1880, depending on the dataset). Reanalyses only extend back to about 1950, but they show more rapid warming over the last 30 years, up to 0.25 C/decade. Satellites do not measure surface temperatures and only begin in 1979, but the two major satellite datasets show warming warming trends exceeding 0.2 C/decade (RSS) and about 0.15 C/decade (UAH) for the last 30 years. Satellites do not yet have accuracy comparable with surface thermometers, but all these confirm that the globe is warming. The empirical data is clear; GMST is about 1.2 C warmer than the 1850-1900 mean, and the rate of change in GMST is accelerating.
But is this unusual? The data say yes. Multiple studies[2][3] show that over the last 2000 years, global temperatures have been comparably stable - a little warmer during the Medieval Warm Period and a little cooler during the Little Ice Age (this is especially true in regional studies). But globally, GMST has increased beyond anything we've seen in the last 2000 years by the mid-20th century and the rate of change has been increasing dramatically; the rate of change is unprecedented during this time frame.
If we extend this to the last 12,000 years[4][5] or even 24,000 years[6], this does not change much. Of the major reconstructions of Holocene temperatures, the reconstruction with the most variability is Marcott 2013.[4] Yet This study shows the warmest point in the Holocene to be significantly cooler than current temperatures, while again rates of change in current warming is unprecedented during this time frame.
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| Pages2K Reconstruction |
We have to go back to the last interglacial to get temperatures potentially warmer than current temperatures,[7] though with CO2 levels at > 400 ppm, we're heading into territory that we've not seen in the Quaternary. The last time CO2 was 400 ppm was about 3 million years ago,[8][9] when sea levels were about 15 m higher and Greenland supported only an ephemeral ice sheet. Nothing I'm saying here is particularly controversial. It's supported by the instrumental record and growing data from proxy evidence that confirm that current warming is unique to the Quaternary, with rates of temperature change possibly unprecedented in geologic history - the only candidates being extreme events like the PETM and end-Permian extinction.[10]
Global Warming is Man-Made
Not only is current global warming exceptional, it's also manmade. In fact the IPCC, following peer-reviewed studies,[11] estimates that virtually all the warming above the 1850-1900 mean is human-caused. This does not mean that there weren't some warming events that were predominantly natural. For instance, natural processes may have been responsible for some of the 1940's warmth, but aerosol pollution following World War II imposed a cooling influence that lowered global temperatures slightly. Over the entire history since 1950, though, natural forcings have combined to produce negligible effects on global temperatures, while anthropogenic forcings explain all the increase in global temperatures. We can determine that this warming is caused by human activity by following a multistep argument. This can be somewhat involved, so I will link to other posts that add context and support to what I'm saying here. Also, please consult the peer-reviewed resources referenced at the bottom of this post so you can check up on everything I say. Here's my multistep argument that virtually all global warming is human-caused:
Increasing atmospheric CO2 concentrations pushes climate towards warming. There is no longer any scientific debate over whether CO2 is a greenhouse gas or whether increasing concentrations of atmospheric CO2 will push climate towards warming. This was first observed experimentally by John Tyndall in the UK in the 1850s, though in controlled lab experiments. Multiple studies show a strong correlation between global temperature and the radiative forcing from CO2. More recently, experiments have observed the greenhouse effect at the surface.[13] It has also been observed with satellites at the top of the atmosphere.[14][15] In a previous post, I summarized another study that successfully showed causation in recent warming from empirical data for CO2 and global temperatures.[12] This study successfully resolved the obstacles associated with inferring causation from correlation by using information flow. The evidence here concerning the effect of increasing CO2 in the atmosphere is uncontested in the scientific literature.
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| The Greenhouse Effect as Seen by Satellites over the Tropical Western Pacific. Dips in the curve correspond to the absorption of IR light at wavelengths associated with GHGs |
2. Humans are responsible for virtually all the increase in CO2 above preindustrial levels.[16] This point is also uncontested by those familiar with the evidence. CO2 has varied between 170 ppm and 300 ppm throughout the Quaternary. Yet right at the same time that we started emitting carbon into the atmosphere, CO2 started to increase, and it's increased precisely at rates corresponding to the rates we've emitted CO2. The natural carbon cycle is nearly balanced, though it removes slightly more CO2 each year than it adds. So human emissions account for virtually all the increase in CO2 above 280 ppm.
3. Natural forcings have had a negligible impact on warming above the 1850-1900 mean.[11] The dominant natural forcings are solar forcings and volcanic eruptions, but these forcing are small compared to anthropogenic forcings. Volcanic eruptions generally cause significant short term cooling, but the cooling only lasts for a couple years or so. Solar variability is cyclical and over the last 170 years its influence has been negligible.
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| Solar Forcings have been Negligible Since 1850 |
4. Warming has been dominated by anthropogenic forcings (CO2,other GHGs and aerosols).[11][17][18][19] CO2 forcings have exceeded 2 Wm^2 while other GHGs add about another 1 W/m^2. Aerosol pollution exerts a cooling influence that removes most of the impact of the other GHGs outside CO2. Total anthropogenic forcings total about 2.2 W/m^2. There are no natural forcings exerting an influence on global climate that is anywhere near this large. The best scientific evidence we have shows that human activity is responsible for virtually all the warming since the late 19th century.
Global Warming is Dangerous
In some ways, the standard of "dangerous" for AGW is a pretty low bar. Any disruption of stability is increases our risk of harm, so any action that is likely to cause disruption is risky. And it's precisely the concept of "risk of harm" that allows us to state that an action is dangerous. As risk of harm increases above levels we consider safe, we call that "dangerous." By way of example, consider driving a car. We generally consider it "safe" to drive a well-built car in good repair on US roads while sober and using safety features like seat belts and air bags. There is of course a risk of harm from accidents that can never be eliminated, but the risk is low enough that we say driving under these conditions is "safe". But some behaviors increase risk of harm. It's "dangerous" to drive while intoxicated because that increases the risk of an accident. Likewise, it's "dangerous" to drive without our seatbelts buckled because if an accident does occur the risk of bodily harm goes up dramatically.
The cost of limiting risk and the value of what we want to protect are also factors. All homeowners accept a risk that their homes may burn down in a fire. We account for that risk by purchasing fire insurance. The probability that any home will burn down in the US is perhaps < 2%. But our homes are extremely valuable to us, and so we pay good money for the insurance, despite the fact that very few of us will ever have to file a claim. The loss of a home from fire would be catastrophic for most families, so even though risk of a fire is low, insurance is worth the cost. In a car accident, decelerating from 70 mph to 0 mph in a few feet can be catastrophic to the passengers of the vehicles involved. The sudden destruction of a home due to a fire can also be catastrophic to our bank accounts without insurance, not to mention the threat to our lives and health. Disruptions of normal increases the risk of harm. Actions that increase the likelihood of these disruptions are dangerous.
We know that human activity is disrupting climate. Global temperatures are rapidly increasing due our activity, and the rate of warming is accelerating. So AGW is almost by definition "dangerous." Our actions are causing disruption that is increasing our risk of harm. And the chance that humanity will experience net harmful effects from AGW is far greater than 2%. It's by far the most likely consequence of our impact on climate. We can say more than "AGW is dangerous." We can say it's likely to cause harm.
Likely Effects of AGW
We cannot be exhaustive here in a single blog post, but we can summarize some of the likely effects of AGW where attribution to AGW is the most clear.
1. Extreme Heat. As we've seen, human activity is responsible for virtually all the 1.2 C global warming above the 1850-1900 mean. The IPCC estimates that we need to stay below 1.5 C warming remain relatively safe from many of the negative effects of AGW. However, we've already seen in a previous post this is goal is nearly unattainable, as is the goal of remaining under 2 C. There is enough future warming baked into current CO2 levels, given an energy imbalance of 0.8 W/m^2 to make 2 C warming inevitable without drastic action in the next couple decades. Warming is likely to reach between 2.5 C and 4 C at or before 2100, depending on what we do between now and then, though should be achieve net zero emissions, GMST may begin to cool.
The inevitable result of global warming is extreme heat. In another post I documented a few studies showing that as GMST increases, the probability of experiencing extreme heat events increases dramatically.[20][21] In fact, looking at seasonal average, if we compare the frequency 3-sigma summers in the 1951-1980 mean (about 0.1% of the globe), those summers cover 22.1% of the globe today - that is, extreme summers are 221% more likely now than they were in 1951-1980. Likewise, heat waves are becoming increasingly more common and severe globally.[22] And this also true in the U.S. A lot of attention is given to the extreme heatwaves during the 1930s "dust bowl" years, and by all accounts these heat waves were severe, but they have been eclipsed by heat waves in recent decades. In another post, I summarized a compelling argument to this effect from Dr. Dessler.
The continued increases in GMST with continued increases in extreme heat is also likely to have an impact on mortality rates. Some recent studies show that direct deaths from hot weather are far outnumbered by deaths due to cold, and many times these studies are cited as evidence that warming will decrease mortality. However, this grossly misunderstands those studies. For one thing, direct deaths to extreme weather is a relatively small factor when compared to increased mortality indirectly related to global warming. And secondly, just because cold weather kills more than hot weather currently does not mean that warming temperatures will produce fewer deaths. We must consider how mortality are likely to change with AGW. For instance, if currently cold weather kills 10,000 people annually, while hot weather kills just 1,000 people annually, the ratio of cold:hot mortality is 10:1. If 1 C warming decreases cold weather deaths to 9,000 people annually but increases hot weather deaths to 3,000 annually, then cold deaths still out number hot deaths - the cold:hot ratio is 3:1, but there still is a net increase of 1000 deaths annually due to climate change. And a recent study[22] shows that this is what we can expect with global warming. "When the reduced sensitivity to heat associated with rising incomes, such as greater ability to invest in air conditioning, is accounted for, the expected end-of-century increase in the global mortality rate is 1.1% [95% CI 0.4-1.9%] in RCP 4.5 and 4.2% [95% CI 1.8-6.7%] in RCP 8.5."[23]
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| Increased Mortality as a Function of GMST |
2. Sea Level Rise. Sea level rise is a well-documented consequence of global warming. Global mean sea levels (GMSL) rise because 1) the thermal expansion of the oceans as ocean temperatures increase and 2) the melting of glacial ice from Greenland, Antarctica and mountain glaciers that flow into the oceans. As global warming has increased, sea level rise (SLR) has been accelerating. Current projections based on observed trends in SLR show that we're on pace to experience 0.6 m or 2 ft of SLR by 2100, assuming that there is no collapse that would cause a more sudden pulse of SLR.[24]
It's important to refer to GMSL here because in any given location, SLR rates may be significantly affected by vertical land movement (VLM), ocean currents, tidal and even gravitational influences. The higher latitudes are still rebounding from the last glacial period, so SLR is not as likely to be an issue nearer to the North Pole. Changes in deposition rates and settling can affect local SLR rates as well. So individual tide gauges say very little about what's happening globally. But all estimates of SLR globally agree that SLR is happening and the rate is accelerating.
And while it may seem like 0.6 m shouldn't be that big of a deal, in point of fact, many living in coastal areas would be significantly affected by this much SLR, and some areas are already effected. Miami is already dealing with the effects of sea level rise, as storm flooding of Miami's streets are now a regular occurrence. Coastal areas in wealthier areas will have to adapt by building infrastructure to limit the effects of SLR, relocating inland, and paying more for insurance. As sea levels rise, tropical storms will be able to do more damage farther inland than they otherwise would.[25] In poorer areas, 0.6 m of SLR will cause millions of climate refugees moving inland, and those inland areas will have to accommodate the influx of refugees.[26]
3. Extreme Weather. AGW is a product of more energy entering the climate system than can escape into space. Some of that energy warms the atmosphere; some melts ice; some heats the ground, but 93% of the energy warms the oceans. As such, we would expect that extra energy to have an impact on weather - there is simply more energy in the climate system to power extreme events. And this is what is observed; we can see this most clearly in the extreme heat and sea level rise above. And as increases in atmosphere GHG concentrations warm the atmosphere, the atmosphere can hold more water vapor, so warmer climates will, on average, permit more rainfall around the globe, increasing risk of flooding in areas that are prone to flooding. Tropical cyclones would thus likely become more intense and be associated with increased precipitation. However, in areas where droughts are more common, warmer temperatures increases vapor pressure deficit (VPD) - if the atmosphere can hold more water vapor but the water isn't there, soils dry out and drought conditions become more likely, as does the risk of wildfire. In the most recent AR6 report,[27] the IPCC summarized the likelihood of these trends in extreme events and the confidence that these increases can be attributed to human-induced climate change.
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| Confidence and likelihood of past changes and projected future changes at 2°C of global warming on the global scale |
There is much more that can be said here, and regional differences will be significant. Some areas will be more prone to flooding while others will be more prone to drought. I'm planning to deal with some of this in more detail in future posts. I've already covered heatwaves, extreme heat and US wildfire statistics and attribution to some extent. Trends in tropical cyclones are covered in more detail in a separate post. In a too-brief summary, AGW is expected to have the following effects on TC trends. TCs are likely to:
a. Become less frequent due to wind shear.b. Become more intense (a higher proportion of cat. 3+ storms) due to warmer SSTs.c. Produce more precipitation due to more atmospheric water vapor.d. Move poleward due to warmer atmospheric temperatures.e. Do more damage inland due to higher sea levels.
4. Greening. One benefit claimed for AGW is that with increasing atmospheric CO2 there will be a "greening" of the earth. And it is generally true that increases in atmospheric CO2 and H2O photosynthesis has more "food" to grow biomass. And greening has been observed in satellite observations. One important 2016 study found that leaf area index (LAI) has increased over 25% to 50% of global vegetative area (greening), while LAI decreased over less than 4% (browning), and CO2 fertilization effects (CFE) explain about 70% of that observed greening trend.[29] The study also examined the impact of climate change on LAI and found mixed results. Climate change explained about 8 ± 20% of the positive trend in LAI. Notice the confidence interval is greater than the estimate. The study found that climate change may be positively impacting northern high latitudes and the Tibetan Plateau, since warmer temperatures leads to longer growing seasons. However, climate change had a negative impact on LAI in South America, which is bad news for the Amazon rainforest.
However, this is not the end of the story. One study published in 2020 provides evidence that the CO2 fertilization effect (CFE) has declined in recent decades between 1982 and 2015.[30] The causes in this decline in CFE are likely due to nutrients and soil moisture limiting photosynthesis. This suggests that we can’t assume that “greening” trends will continue as they have. And it also suggests that models are over predicting how much CO2 will be taken up by plant life as CO2 increases and global warming continues. A declining CFE will leave more CO2 in the atmosphere to warm the planet. The negative feedback appears to be diminishing.
Another study suggests that the “greening” trend may have already slowed or stopped.[31] The study finds that vapor pressure deficit (VPD) increased sharply following the late 90s, and “greening” has stalled since then. It indicates that increased VPD began to offset CFE over the last 20 years. The bad news here is that “six Earth system models have consistently projected continuous increases of VPD throughout the current century.” So as VPD increases, it may well offset the benefits of CFE.
Still another study published in 2020 examined trends in seasonal forests in Brazil and finds that these forests are moving from a net carbon sink to a carbon source likely because of an increase in tree mortality.[32] The study only examined areas that were “free from fire, flood, landslides, and human disturbances at least for decades before the first census of each site.” The study found “decreasing carbon gains (2.6% by year) and increasing carbon losses (3.4% by year).” Some of the warmest and driest sites have already transitioned to a net carbon source. In other words, these Brazilian forests may be transitioning into a carbon source instead of a carbon sink.
5. Crop Yields. In a similar manner to "greening," the effect of AGW on crop yields is likely to have a mixed bag of results. Many studies show increased CO2 benefits crops grown in greenhouses, since in greenhouses you can control for threats to crop growth. But studies examining crops under real life conditions are showing that the benefits of CO2 also benefit weeds, pests, and disease, and these can outcompete crops and limit the response of crops to increased CO2.[33] And the increased yield from increased CO2 is basically all carbs. It does not increase other nutrients, so you can end up with CO2-induced deficits of protein vitamins and minerals.[34] Here are some general expectations of the effects of AGW on crop yields that come from the NCA4 report:[35]
a. Long-term temperature increases can reduce future yields of commodity crops like corn, soybean, wheat, rice, cotton, and oat. [36-42]c. In the US, higher temperatures are expected to cause a decline in commodity crop yields. [44-46]
b. Warmer temperatures may improve yields for wheat, hay, and barley.[43]
With all these dangers to AGW with even the so-called positive impacts of AGW being a mixed bag of results, there is a compelling case to replace the activities that cause AGW (primarily fossil fuel use) with activities that do not (nuclear, renewables and other clean energy sources). At the same time, we cannot deny the overwhelming benefits that fossil fuels have had in our lives. They have provided the energy we have needed to improve our way of life in countless ways. The impression some may get from all I've said so far might be that fossil fuels are evil, but that's simply not the case. By way of analogy, we can think of fossil fuels as a kind of medication that saves lives but over time side effects build up that make it not effective in terms of long term care. As the side effects build up, eventually the benefits of the medication are outweighed by its risks, and another medication with fewer side effects is required. Nobody disputes massive positive impact fossil fuels have had for humanity, but side effect of greenhouse gas warming are building up so that we need to to transition cleaner sources of energy. Fossil fuels are also the most harmful source of energy we have, causing over 8 million deaths annually due to air pollution,[47] while nuclear and renewables are by far the safest forms of energy available to us.[48] The cost of the side effects are going to become to great, given alternatives.
6. Tipping Points. There has been much written on the effects of tipping points in recent years, and not all of it has been helpful. Tipping points refer to thresholds within various features of the climate system that, when crossed, lead to to irreversible effects. These effects may or may not be severe, and they may or may not have a large impact on the climate system. Some treat tipping points as a point beyond which catastrophic runaway warming will occur, such that mitigation will no longer do any good - that is a sort of "cliff" that once you fall off it, there's nothing that can happen until you go splat at the bottom. The evidence does not support this. But the evidence does support that tipping points may be a type of steep slope, such that once we start going down it, it's extremely difficult to climb back up. Tipping points are an argument for urgency - we save ourselves a lot of work and expense if we can avoid crossing these thresholds. I have a summary of the evidence for this in another post.
Mitigation and Adaptation
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[48] "What are the safest and cleanest sources of energy?"
Excellent! Very informative and well written. Thank you!
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