From
BAUT
Many thanks Trakar, this is an important paper. I have read it today and have the following comments.
The rate of human made change of atmospheric CO2 amount is now much larger than slow geological changes. Humans now determine atmospheric composition, for better or worse, and they are likely to continue to do so, as long as the species survives.
• We have moved from the Holocene to the Anthropocene. Humans are not above nature or separate from it, but part of it. Global regulation of atmospheric composition is a key to global economic security and stability. We have no choice but to embark on massive geo-engineering, on the scale of the Manhattan and Apollo Projects, based on scientific assessment of methods that will be most environmentally and economically beneficial.
The Cenozoic Era helps us determine the dangerous level of human-made climate Change
• Hansen studies the whole record since the dinosaurs, and finds that the climate is far more sensitive to small forcing than is generally understood. What we are doing now is extremely dangerous.
Milankovitch (1941) suggested that these climate swings occur in association with periodic perturbations of Earth's orbit by other planets (Berger, 1978) that alter the geographical and seasonal distribution of insolation over Earth's surface. The varying orbital parameters are (1) tilt of Earth's spin axis relative to the orbital plane, (2) eccentricity of Earth's orbit, (3) day of year when Earth is closest to the sun, also describable as precession of the equinoxes (Berger, 1978).
• To clarify, point three is precession of the equinoxes combined with procession of the perihelion axis. This is why this cycle is about 21,600 years in the climate record whereas precession is a cycle close to 25,800 years.
a target of 2°C for limiting human-made climate change is too high … paleoclimate data on climate change and climate sensitivity can be pushed further to yield an accurate evaluation of the dangerous level of global warming. Broad-based assessments, represented by a "burning embers" diagram in IPCC (2001, 2007), suggested that major problems begin with global warming of 2-3°C relative to global temperature in year 2000…. paleoclimate data imply that 2°C global warming would be a disaster scenario for much of humanity and many other species on the plane
• The geological record shows that when the world was 2°C warmer the sea was much higher. Accepting such a target as inevitable means we are storing up ‘tectonic’ pressure, with the current stability likely to have inertia, in view of the extreme rapidity of the change, but to risk shift to a new stability extremely suddenly, with collapse of Antarctic and Greenland ice sheets.
We interpret the stability of Holocene sea level as a consequence of the fact that global temperature was just below the level required to initiate the "albedo flip" mechanism on the fringes of West Antarctica and on most of Greenland. An important implication of this interpretation is that the world today is on the verge of, or has already reached, a level of global warming for which the equilibrium surface air temperature response on the ice sheets will exceed global warming by much more than a factor of two.
• This shows the key importance of thresholds, and the immense risk that stepping across a threshold puts the planet into a completely different space. We have already reached a state where the equilibrium is across a threshold, but the move has not yet happened.
today's climate models generally are less sensitive to forcings than the real world (Valdes, 2011), suggesting that models do not capture well some amplifying climate feedbacks and thus making empirical assessment via Earth's history of paramount importance.
• Prediction needs to be based much more systematically on comparison to what actually happened in the past. Getting all the factors into computer climate models is immensely complex. I thought some models were more sensitive to forcing than the real world, so it is interesting that Hansen says this is not the case.
Sea level rise, despite its potential importance, is one of the least well understood impacts of human-made climate change. The difficulty stems from the fact that ice sheet disintegration is a complex non-linear phenomenon that is inherently difficult to simulate, as well as from the absence of a good paleoclimate analogue for the rapidly increasing human-made climate forcing.
• This non-linearity makes climate change as hard to predict as the timing of earthquakes. However, like plate tectonics, we know that eventually something has to give when massive forces are in slow collision. The exact timing is uncertain, but the eventual occurrence is certain.
equilibrium (eventual) sea level change in response to global temperature change is about 20 meters for each degree Celsius global warming … eventual sea level rise of several tens of meters must be anticipated in response to the global warming of several degrees Celsius that is expected under business-as-usual (BAU) climate scenarios
• This is an amazing claim. It means we are storing up big storms for our grandchildren. It shows the expected rise of four degrees this century is not just the difference between tropical and temperate climate, but more akin to the difference between normal body temperature and a raging fever.
the fundamental issue is linearity versus non-linearity. Hansen (2005, 2007) argues that amplifying feedbacks make ice sheet disintegration necessarily highly non-linear, and that IPCC's BAU forcing is so huge that it is difficult to see how ice shelves would survive. As warming increases, the number of ice streams contributing to mass loss will increase, contributing to a nonlinear response that should be approximated better by an exponential than by a linear fit. Hansen (2007) suggested that a 10-year doubling time was plausible, and pointed out that such a doubling time, from a 1 mm per year ice sheet contribution to sea level in the decade 2005-2015, would lead to a cumulative 5 m sea level rise by 2095.
• Yet even the linear data of global warming is alarming. It makes sense that the extreme speed of change in geological terms will produce big lags in climate response, and that when the lags hit they will do so with immense force.
Pfeffer at al. (2008) … for Greenland …assume that ice streams this century will disgorge ice no faster than the fastest rate observed in recent decades. That assumption is dubious, given the huge climate change that will occur under BAU scenarios, which have a positive (warming) climate forcing that is increasing at a rate dwarfing any known natural forcing. BAU scenarios lead to CO2 levels higher than any since 32 My ago, when Antarctica glaciated. By mid-century most of Greenland would be experiencing summer melting … The main flaw with the kinematic constraint concept is the geology of Antarctica, where large portions of the ice sheet are buttressed by ice shelves that are unlikely to survive BAU climate scenarios. West Antarctica's Pine Island Glacier (PIG) illustrates nonlinear processes already coming into play … there is sufficient readily available ice to cause multi-meter sea level rise this century, if dynamic discharge of ice increases exponentially. Thus current observations of ice sheet mass loss are of special interest … data records suggest that the rate of mass loss is increasing, indeed nearly doubling over the period of record, but the record is too short to provide a meaningful evaluation of a doubling time
• Again, modelling of non-linear processes is essential, while recognising the chaotic trigger factors sit within a larger framework that is definitely linear. CO2 levels higher than an ice-free world will eventually cause an ice-free world.
Earth in the warmest interglacial periods of the past million years was less than 1°C warmer than in the Holocene. Polar warmth in those interglacials and in the Pliocene does not imply that a substantial cushion remains between today's climate and dangerous warming, but rather that Earth is poised to experience strong amplifying polar feedbacks in response to moderate additional global warming.
• This analysis of amplifying feedbacks and the lack of cushion shows the extreme sensitivity of global climate to relatively small forcing factors, like billions of tons of CO2
burning all or most fossil fuels guarantees tens of meters of sea level rise, as we have shown that the eventual sea level response is about 20 meters of sea level for each degree Celsius of global warming. We suggest that ice sheet disintegration will be a nonlinear process, spurred by an increasing forcing and by amplifying feedbacks which is better characterized by a doubling time for the rate of mass disintegration, rather than a linear rate of mass change. If the doubling time is as short as a decade, multi-meter sea level rise could occur this century. Observations of mass loss from Greenland and Antarctica are too brief for significant conclusions, but they are not inconsistent with a doubling time of a decade or less. The picture will become clearer as the measurement record lengthens
• Illustrates the previous point, adding emphasis on the need to shift to a non fossil fuel economy. My opinion is that large scale ocean based algae biofuel and food production is the best rapid option to regulate the global climate through geoengineering in a way that will provide a feasible transition path by retaining the use of liquid fuel.
High latitude cooling and low latitude warming would drive more powerful mid-latitude cyclonic storms, including more frequent cases of hurricane force winds. Such storms, in combination with rising sea level, would be disastrous for many of the world's great cities and they would be devastating for the world's economic well-being and cultural heritage.
• Sudden tipping of ice sheets into the ocean could cool the poles while the equator heats up, generating massive imbalances.
a target of 2°C is not safe or appropriate. Global warming of 2°C would make Earth much warmer than in the Eemian, when sea level was 4-6 meters higher than today. Indeed, with global warming of 2°C Earth would be headed back toward Pliocene-like conditions. Conceivably a 2°C target is based partly on a perception of what is politically realistic, rather than a statement of pure science. In any event, our science analysis suggests that such a target is not only unwise, but likely a disaster scenario.
• This illustrates the need to look at what fixtheclimate.org has said about the failure of the ‘emission reduction’ model through market signals to be a sufficient measure to address the climate crisis. We rather urgently need to geoengineer as the prime security issue for our planet. The ‘realism’ is based on existing technology, when it is very likely that large resources thrown at the problem can produce new technology fast.
atmospheric CO2 should be rolled back from its present ~390 ppm at least to the level of approximately 350 ppm. With other climate forcings held fixed, CO2 at 350 ppm would restore the planet's energy balance and keep human-made global warming less than 1°C,
• And that needs new technology. My estimate is that algae farms on 0.1% of the world ocean could do it.