You may have heard that recently someone in the UK challenged a school district in court to try and block them from showing Al Gore’s Oscar winning documentary to the students. This court challenge was based on a set of 9 alleged “errors” in the movie.
And now the excellent and most credible RealClimate website offers their opinion of those 9 alleged “errors”:
- Ice-sheet driven sea level rise Gore correctly asserted that melting of Greenland or the West Antarctic ice sheet would raise sea levels 20ft (6 meters). In the movie, no timescale for that was specified, but lest you think that the 20 ft number is simply plucked out of thin air, you should note that this is about how much higher sea level was around 125,000 years ago during the last inter-glacial period. Then, global temperatures were only a degree or two warmer than today – and given that this is close to the minimum temperature rise we can expect in the future, that 20 ft is particularly relevant. The rate at which this is likely to happen is however highly uncertain as we have discussed previously.
- Pacific island nations needing to evacuate Much of Tuvalu is only a few feet above sea level, and any sea level rise is going to impact them strongly. The impacts are felt in seemingly disconnected ways – increasing brine in groundwater, increasing damage and coastal erosion from tides and storm surges, but they are no less real for that. The government of Tuvalu has asked New Zealand to be ready to evacuate islanders if needed, and while currently only 75 people per year can potentially be resettled, this could change if the situation worsened.
In the movie there is only one line that referred to this: “That’s why the citizens of these pacific nations have all had to evacuate to New Zealand”, which is out of context in the passage it’s in, but could be said to only be a little ahead of it’s time.
- Climate impacts on the ocean conveyor The movie references the Younger Dryas event that occurred 11,000 years ago when, it is thought, a large discharge of fresh water into the North Atlantic disrupted the currents, causing significant regional cooling. That exact scenario can’t happen again, but similar processes are likely to occur. The primary unresolved scientific issue regards how quickly the circulation is likely to change as we move forward. The model simulations in the latest IPCC report show a slowdown in the circulation – by about 30% by 2100 – but there is much we don’t understand about modeling that circulation and future inputs of freshwater from the ice sheets, so few are willing to completely rule out the possibility of a more substantial change in the future. Further discussion on what this really means and doesn’t mean is available here and here.
- CO2 and Temperature connections in the ice core record Gore stated that the greenhouse gas levels and temperature changes over ice age signals had a complex relationship but that they ‘fit’. Again, both of these statements are true. The complexity though is actually quite fascinating and warrants being further discussed by those interested in how the carbon cycle will react in the future. We’ve discussed the lead/lag issue previously. A full understanding of why CO2 changes in precisely the pattern that it does during ice ages is elusive, but among the most plausible explanations is that increased received solar radiation in the southern hemisphere due to changes in Earth’s orbital geometry warms the southern ocean, releasing CO2 into the atmosphere, which then leads to further warming through an enhanced greenhouse effect. Gore’s terse explanation of course does not mention such complexities, but the crux of his point–that the observed long-term relationship between CO2 and temperature in Antarctica supports our understanding of the warming impact of increased CO2 concentrations–is correct. Moreover, our knowledge of why CO2 is changing now (fossil fuel burning) is solid. We also know that CO2 is a greenhouse gas, and that the carbon cycle feedback is positive (increasing temperatures lead to increasing CO2 and CH4), implying that future changes in CO2 will be larger than we might anticipate.
- Kilimanjaro Gore is on even more solid ground with Kilimanjaro. In the movie, the retreat of Kilimanjaro is not claimed to be purely due to global warming , but it is a legitimate example of the sort of thing one expects in a warmer world, and is consistent with what almost all other tropical mountain glaciers are doing. There is indeed some ongoing discussion in the literature as to whether or not the retreat of ice on Kilimanjaro is related to the direct effects (warming atmospheric temperatures) or indirect effects (altered patterns of humidity, cloud cover, and precipitation influencing Kilimanjaro’s ice mass) of climate change, and that argument isn’t yet over. But these arguments would be of more relevance if (a) we were not witnessing the imminent demise of an ice field that we know has existed for at least the past 12,000 years and (b) most of the other glaciers weren’t disappearing as well.
- Drying up of Lake Chad It is undisputed that Lake Chad has indeed shrunk rapidly in recent decades. While irrigation and upstream water use are probably contributing factors, the dominant cause is the reduction of rainfall across the entire Sahel from the 1950s to the 1980s and with rainfall today still substantially below the high point 50 years ago. There is substantial evidence that at least a portion of this drying out is human-caused. A few recent papers (Held et al, PNAS; Chung and Ramanathan and Biasutti and Giannini) have addressed causes ranging from Indian Ocean changes in sea surface temperature to the increase in atmospheric aerosols in the Northern hemisphere. Gore uses this example to illustrate that there are droughts in some regions even while other areas are flooding. Unfortunately this is exactly what the models suggest will happen.
- Hurricane Katrina and global warming Katrina is used in the film as a legitimate illustration of the destructive power of hurricanes, our inability to cope with natural disaster, and the kind of thing that could well get worse in a warmer world. Nowhere does Gore state that Katrina was caused by global warming. We discussed this attribution issue back in 2005, and what we said then still holds. Individual hurricanes cannot be attributed to global warming, but the statistics of hurricanes, in particular the maximum intensities attained by storms, may indeed be.
- Impact of sea ice retreat on Polar bears As we presaged in August, summer Arctic sea ice shattered all records this year for the minimum extent. This was partially related to wind patterns favorable to ice export in the spring, but the long term trends are almost certainly related to the ongoing and dramatic warming in the Arctic. Polar bears do indeed depend on the sea ice to hunt for seals in the spring and summer, and so a disappearance of this ice is likely to impact them severely. The specific anecdote referred to in the movie came from observations of anomalous drownings of bears in 2004 and so was accurate. However, studying the regional populations of polar bears is not easy and assessing their prospects is tough. In the best observed populations such as in western Hudson Bay (Stirling and Parkinson, 2006), female polar bear weight is going down as the sea ice retreats over the last 25 years, and the FWS is considering an endangered species listing. However, it should be stated that in most of the discussions about polar bears, they are used as a representative species. Arctic ecosystems are changing on many different levels, but it is unsurprising that charismatic mega-fauna get more press than bivalves. In the end, it may be the smaller and less photogenic elements that have the biggest impact.
- Impact of ocean warming on coral reefs Corals are under stress from a multitude of factors; overfishing, deliberate destruction, water pollution, sea level rise, ocean acidification and, finally, warming oceans. The comment in the movie that rising temperatures and other factors cause coral bleaching is undoubtedly true. Bleaching episodes happen when the coral is under stress, and many examples have been linked to anomalously warm ocean temperatures (Australia in 1998 and 2002, all over the Indian Ocean in recent years). Corals are a sobering example of how climate change exacerbates existing vulnerabilities in eco-systems, potentially playing the role of the straw that breaks the camel’s back in many instances.
Overall, our verdict is that the 9 points are not “errors” at all (with possibly one unwise choice of tense on the island evacuation point). But behind each of these issues lies some fascinating, and in some cases worrying, scientific findings and we can only applaud the prospect that more classroom discussions of these subjects may occur because of this court case.