(all quotes from the summary for policymakers)
In this report, the term impacts is used primarily to refer to the effects on natural and human systems of extreme weather and climate events and of climate change
Or in other words, they are lumping together costs associated with climate change due to human action, costs associated with climate change from other causes, and costs associated with extreme climate events, whether or not due to climate change.
In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. ... Evidence of climate change impacts is strongest and most comprehensive for natural systems. ... See supplementary Table SPM.A1 for descriptions of the impacts (B) Average rates of change in distribution (km per d ecade) for marine taxonomic groups ... . Positive distribution changes are consistent with warming (moving into previously cooler waters, generally poleward).
"Impacts" sounds scary, but, as the quote shows, only means change.
While only a few recent species extinctions have been attributed as yet to climate change (high confidence), natural global climate change at rates slower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years.
Warnings about large numbers of species being driven to extinction by anthropogenic climate change have morphed into the observation that species have gone extinct in the past for reasons unrelated to human action.
Some low-lying developing countries and small island states are expected to face very high impacts that, in some cases, could have associated damage and adaptation costs of several percentage points of GDP.
Compare costs of "several percentage points of GDP" in the places most at risk due to sea level rise with past rhetoric of hundreds of millions of climate refugees, drowned island chains, and the like.
Climate change has negatively affected wheat and maize yields for many regions and in the global aggregate (medium confidence). Effects on rice and soybean yield have been smaller in major production regions and globally, with a median change of zero across all available data
They don't actually say that yields have fallen, although that is what a careless reader is likely to think they are saying, only that they are lower than they would have been without climate change. A little searching finds a scholarly article on wheat yields, published in 2012, which reports that
Wheat yields have increased approximately linearly since the mid-twentieth century across the globe, but stagnation of these trends has now been suggested for several nations. .... With the major exception of India, the majority of leveling in wheat yields occurs within developed nations—including the United Kingdom, France and Germany—whose policies appear to have disincentivized yield increases relative to other objectives. The effects of climate change and of yields nearing their maximum potential may also be important.
Near the time that leveling is generally observed, the European Union shifted away from a policy that rewarded high agricultural production through price guarantees to a policy that pays flat subsidies that do not increase with production and triggers taxes when production limits are exceeded
So what has actually happened is not that yields have decreased but that in some areas they have stopped increasing, at least in part due to changes in agricultural policy.
At present the world-wide burden of human ill-health from climate change is relatively small compared with effects of other stressors and is not well quantified. However, there has been increased heat-related mortality and decreased cold-related mortality in some regions as a result of warming ... .
The first sentence makes it sound as though climate change is making things worse. The second implies that there have been both costs and benefits and offers no estimate of their relative size.
People who are socially, economically, culturally, politically, institutionally, or otherwise marginalized are especially vulnerable to climate change.
One might conclude that keeping poor people poor, for instance by pressuring poor countries to produce less energy or produce it in more expensive ways in order to hold down CO2 output, will do more damage than good.
Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability.
Note that the extremes are not limited to those due to climate change. Floods, cyclones, et. al. do damage...and always have.
For the major crops (wheat, rice, and maize) in tropical and temperate regions, climate change without adaptation is projected to negatively impact production for local temperature increases of 2°C or more above late-20th-century levels, ...
Emphasis mine. If farmers ignore the implications of climate change on what crops they should grow how and continue to ignore them for the next sixty years or so, output is expected to decline.
With these recognized limitations, the incomplete estimates of global annual economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0% of income (±1 standard deviation around the mean)
I have not yet gotten into the full report but, judging from accounts I have seen, 2°C of additional warming is about what it suggests we can expect by 2100 if we don't do much to prevent it. So if policies to prevent warming reduce the annual growth rate of world income from (say) 2% to 1.98%, the resulting loss will just about cancel the gain. Not a compelling argument for switching from fossil fuels to solar power.
All of these quotes are from the first half of the summary for policy makers—I have not yet tried to get into the full report. My conclusion is that the IPCC's estimates of the negative effects of climate change due to human action are much smaller than the rhetoric surrounding the subject suggests, a fact the report attempts to conceal as best it can by its presentation.
And a fact that does not come through in news stories about the report, at least those I have so far looked at.
How long before the more enthusiastic true believers start accusing the IPCC of having sold out to the oil industry?