What are our risks?

Risk is the key concept in the need for climate change mitigation. There is little doubt that the Earth is warming, and it seems very plausible that it is caused by humans. Regardless of what scientists and models tell us about our future, we really have no idea how our climate system will react to this warming--most importantly, that the system clearly does not react smoothly and has not reacted smoothly throughout its history. Major changes, such as a massive release of carbon dioxide into the atmosphere, have many feedbacks both positive and negative that push and pull the system back and forth. It is certainly possible that our planet may have negative feedbacks that will be triggered in the near future that will counteract the effects of carbon dioxide (and other gas) emissions, and everything will be fine. But nobody knows what will happen for sure, and nobody wants to test our planet to find out. So long as we continue this carbon dioxide experiment, we place tremendous risk on our future--this is completely undeniable.

Future Climate Risks

Again, it's important to emphasize that the following are possibilities (some might even say a few are "improbabilities"). These "improbabilities" are concerns simply because they have occurred due to rapid warming in the geologic past. We really have no idea when they could occur, and if possible, we'd prefer to keep it that way.

1. Possibility of increased frequency and intensity of extreme weather events
   • Floods. Floods affect 150 million people annually
   • Possible increase in Hurricane/Typhoon intensity. Theory shows that hurricanes derive their "fuel" from the warm sea surface temperatures (SSTs), so increasing SSTs would likely imply increased storm intensity. However, the state of the atmosphere around the hurricane is very important in the ability for hurricanes to develop, and these factors may correlate negatively with SSTs--in other words, warmer SSTs help hurricanes but associated changes in circulation hurt them. Furthermore, there have been conflicting studies on recent trends in hurricane activity, and these studies are also subject to bias given that 1) hurricane classification is based upon instrument readings, and this technology has changed over time, and 2) there are many internal cycles on timescales of years, decades, and perhaps longer that may have a much stronger influence on hurricane activity than recent global warming. Nonetheless, the most likely scenario is that there will be an increase in the number of major hurricanes (simply because they have more available energy), and frequency may increase as well simply due to the fact that warmer SSTs implies a wider region where hurricanes can form (there is a minimum SST of 26.5C below which hurricanes very rarely form)
   • Some evidence of increasing severe weather: However, at this point the case is likely far overstated. Most of the science behind this claim involves analysis of recent weather records and trends. However, the fact is that as little as 20 years ago, we didn't have the technology to detect tornadoes and other severe weather events unless a person was actually witness to it (e.g. we can "spot" tornadoes today using doppler radar today, which means the total number of tornadoes reported is much higher). So any trends are likely to have a strong bias, making this claim one of the least well-substantiated in terms of global warming consequences.
2. Rising sea levels: 2/3 of the world's population lives within 50 miles of the coast. Today, Greenland, which constitutes about 10% of the world's ice, is melting rapidly, and Antarctica, which constitutes about 90% of the world's ice, is (luckily) somewhat stable--some studies show it is growing slightly, some show it is melting slightly. In any case, the melting of the entire Greenland ice sheet, which is projected in some climate models to occur within the next 100 years, would cause about a 15 foot sea level rise. For those who remember the effects of the tsunami in Bangladesh, there are millions of people whose lives would be devastated by such a rise (of course, the rise would be gradual, and not in a huge wave. But their former residences would be underwater nonetheless). Sea level rise also occurs in part due to thermal expansion (warmer water takes up more volume than cooler water), which has thus far constituted the majority of the recent sea level rises.
3. Highly variable magnitudes of REGIONAL climate change (temperatures, rainfall etc.): What happens to the agriculture and the ecosystems of a region whose climate warms by 10F and receives only half as much precipitation as in the past?
1. Major agricultural impacts: crop production, yield, quality, diversity. This is very important. If a region has an economy dependent on a particular crop, and suddenly in the span of a few decades it can no longer grow this crop (or even worse, it cannot grow any crop due to soil destruction), there are substantial economic and social consequences as a result. One may point out that, on the other hand, there will be regions (e.g. much of Canada) that was once too cold to grow crops that will be able to in a warmer climate. But the problem is we have NO control over these changes nor should we assume that we understand where, when, and how large these changes will be.
2. Environmental destruction: ecosystem collapses, species extinction because the climate changes are occurring too fast for them to adapt. Again, while to many people the loss of polar bears in the arctic, for example, does not seem important, the fact is that we have no idea how the earth's ecosystems will react to climate shifts on such short time scales. A warming of a few degrees (and more in some regions) can have profound effects on animals and plants, including trees that we are already cutting down at alarming rates. It is possible for our ecosystem to adapt to such changes, but the key question remains: can species adapt fast enough? Again, we depend on our biosphere in many ways that we either do not understand or simply take for granted (e.g. oxygenation, forest growth, food etc.), and so we are naive and downright stupid if we believe that we either can survive without the rest of the ecosystem or can predict how it will be affected by our decisions.
3. Major water shortages: This may turn out to be one of the most pressing consequences of climate change. For example, the U.S. West gets up to 80% of its water supply for cities/crops from melting of the annual mountain snow pack, which is dwindling as the globe warms and mountain snow packs shrink. California, in fact, is already enacting multi-billion dollar initiatives to deal with major water shortages that have occurred over the past decade. The loss of snow and subsequent loss of water supply may be very problematic for both agriculture (irrigation) and society (urban areas need water) that would cost tremendous amounts of money to fix--it would require an entirely new hydrological infrastructure over large areas at incredible costs. Building one dam will not do the trick.
4. Increasing potential (unlikely, but not unreasonable) for one of the “tipping points”: a threshold in the climate system that triggers an irreversible large-scale change
• Gulf Stream Collapse: resulting in a sudden ice age. By "sudden," this likely does not imply within a few days, or even a few years, but instead may take place in a "geological instant" over decades or centuries. However (and this is important), that is absolutely no excuse to assume that this is thus not an important concern, because such a change would likely be triggered early on, and once it occurs, we probably would not be able to stop it. The most common scenario through which an ice age would be triggered is via the shutting down of the thermohaline ocean current that transports heat from the tropics to the mid-latitudes, especially Europe. This current flows along the ocean surface up towards the UK and Greenland and then sinks into the depths of the ocean and returns southward; however, as Greenland ice melts (freshwater) into the ocean current, this dilutes the denser salty current and could potentially prevent it from sinking and thus halting its circulation. It is believed that this has occurred many times in the past, and it usually results in a dramatic drop in temperatures, especially in Europe.
• Collapse of the Amazon Rain Forest Ecosystem: resulting in significant release of CO2 that would considerably increase rate of climate change
• Large-scale sea-floor methane release: Rising ocean temperatures could release beds of frozen methane clathrates from the sea floor, especially along the continental shelves, emitting a tremendous amount of methane into the atmosphere. Given that methane is a very powerful greenhouse gas (much stronger than CO2), but it has a much shorter atmospheric lifetime, for methane to have a huge impact requires a large-scale release that would cause an additional warming of large enough magnitude to further release frozen clathrates (a positive feedback). The Paleocene-Eocene Thermal Maximum (a major sudden warming event) 55 million years ago is believed to have been caused by this.

A Comment on Catastrophic Climate Changes

Much debate has taken place in the public discourse regarding the value of abrupt climate change speculation. Many feel that the concept of catastrophe is used merely as a scare tactic to garner support for the cause and is nothing more than environmental "fearmongering." From what I have read and heard, at some level I do agree that the fears of catastrophe may be overstated, especially by the ratings-driven media. People are under the impression that, for example, an ice age would suddenly occur within a matter of hours or days (as in "The Day After Tomorrow"), and given the seeming improbility of such a scenario, these people then reject the entire science altogether. I believe that this is mostly the fault of the media for exaggerating claims of scientists, but I also fault the scientists somewhat for not being more explicit about the dangers. My guess is that scientists are reluctant to downplay exaggerated concern because the public probably would feel no need to take action if an ice age would not take place for 500 years.

Whether or not this is fair is hard to say, but the fact is that even with a low probability of something catastrophic occurring (especially because these events have occurred in the past under 3-5 degrees of warming), if something irreversible does begin, by the time we recognize it, it will probably be too late to do anything about it. And while 500 years may seem like a lot, its only around 15 generations; I like to think that we should want civilization to survive more than a mere 15 generations into the future.

Overall

Irreversible climate changes aside, future costs to adapt society to a rapidly changing climate (rather than try to stop it) are certainly large but unknown. I can't imagine that any study that claims to have a dollar value on the cost of adapting to climate change (in order, most likely, to show that the costs to reduce CO₂ emissions will be higher than the costs to adapt) really has any idea what they are talking about, especially given the fact that regional climate models on which the studies are based are not incredibly trustworthy. Costs will be incurred at local levels (e.g. farms that must be relocated, dams that need to be built etc.), and we have no idea what the effects of global climate change will be at such a small-scale. All we know is that it will probably be very, very costly.

Overall, can you give me some quick reasoning for why we should act?

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