Climate scientist James Hansen has put forward a succinct guide to current climate science in Climate Change in a Nutshell: The Gathering Storm.
It was written as an expert report for Juliana v United States — an ongoing 4-year court challenge by a group of students against the US government for its role in the climate crisis.
In August 2015, 21 students aged 8 to 19 filed a lawsuit in the Oregon District Court against the US government, then-president Barack Obama and several members of his cabinet.
The students, backed by the non-profit organisation Our Children’s Trust, chose to include in their lawsuits those who had suffered “concrete injury” of floods and drought. The complaint asserts that the government’s actions have, through the impact of the climate crisis, violated the youngest generation’s constitutional rights to life, liberty and property.
Three fossil fuel-related industry groups joined the case as defendants to support the government’s efforts to have the case dismissed. The industry groups withdrew, however, after Donald Trump was elected president — thereby replacing Obama as the lead defendant — as they felt their support was no longer needed.
The defendants’ motions to have the case dismissed were denied by the courts in November 2016 and again in June 2017. However, further attempts to delay and stymie the case put it on hold.
Hansen prepared the expert report last year. Hansen is probably the world’s most renowned climate scientist and has been pointing to the dangers of climate change since the 1980s. He is a former NASA scientist and is now director of a climate science institute at Columbia University. He is also the grandfather of one of the Juliana v United States plaintiffs.
Earth’s energy balance
The crux of the issue, as outlined in the document, is the Earth’s energy balance. The Earth absorbs a certain amount of energy and achieves a balance by radiating some of that energy out again. But the energy balance can be changed by climate forcings, whereby more energy comes in or out, changing the temperature of the Earth until it reaches a new balance at a higher or lower level.
Some of these forcings are natural. For example, the sun’s energy has cycles by which its energy varies by about 0.1%. The Earth normally absorbs 240 watts per square metre (W/m2), so such a change in the sun’s brightness can lead to a forcing of 0.24W/m2.
Then there are human-caused climate forcings, most notably the emission of greenhouse gases (GHG) into the atmosphere.
The absorption of heat radiation by carbon dioxide has been accurately measured and confirmed by science. A rise in the level of carbon dioxide in the atmosphere from the pre-industrial level of 280 parts per million (ppm) to the 2018 level of 407ppm has caused a climate forcing of about 2.1W/m2.
Other GHG emissions equate to a forcing of about 1W/m2, while the emission of aerosols that reflect sunlight have contributed to a negative forcing of about the same degree.
Though sun cycles are a favourite talking point for climate deniers, they are clearly only able to play a minor role in the climate crisis relative to GHG. Also, while they can lead to a rise in the energy balance, they ultimately also contribute to a similar decrease throughout the cycle. GHG have only contributed to a warming.
While calculating the level of climate forcings is relatively easy, working out the impact of forcings is more complex due to climate feedbacks.
Climate feedbacks
Hansen’s report points out that as the Earth warms, so to does water vapour, which is an amplifying feedback as water vapour is a GHG. At the same time, a rise in water vapour can also cause some clouds to thicken and reflect more sunlight.
Unfortunately, the main feedback effects are of the amplifying kind.
The 1979 Charney report into the impact of carbon dioxide on the climate concluded that a doubling of carbon dioxide from 280 ppm to 560 ppm would likely cause a rise of 3°C, with an uncertainty range of 1.5°C. That is a rise of 0.75°C for each W/m2 of climate forcing.
Hansen writes that this calculation of climate sensitivity took account of fast feedback effects but left out slow feedbacks.
One of these is the melting of ice sheets. Ice sheets reflect the sun’s energy. But, as the Earth warms, ice sheets melt, exposing the darker surface underneath. This surface absorbs more energy, leading to more warming.
A second slow, but powerful, feedback is due to the fact that a lot of GHG are kept out of the atmosphere and instead absorbed by the ocean and soil. Warming causes oceans to release carbon dioxide and tundra and wetlands to release GHG, again leading to more warming.
Like the Charney report, most climate models only account for fast feedback effects. This includes those used by the Intergovernmental Panel on Climate Change.
The ocean has especially great inertia in response to climate forcings. Even 100 years after a climate forcing it can be expected only to have reached two-thirds of its new equilibrium temperature. If climate forcing were to stop today, there would still be another 0.5°C of warming in the pipeline.
Role of US and China
Among the other issues that the report covers, such as developments in climate science, impacts on weather events, sea levels and species extinction, is the responsibility of the US for carbon dioxide emissions.
It says that China has overtaken the US in emissions and is now responsible for more than double the emissions of the US. Yet, the report notes that it is cumulative emissions that cause climate change and that the US is far more responsible for cumulative emissions — about a quarter of the total and double that of China.
It also points out that we should look at per capita emissions for a perspective on responsibility for anthropogenic climate change. In this, the US is way ahead of China, based on 2016 per capita emissions — and Australia is ahead of the US.
The gap is even greater if we look at cumulative per capita emissions.
Hansen has condemned the weak response to the climate crisis from governments. He says even some of the more progressive responses are “half-arsed, half-baked plans only to delay a solution”.
Legal actions
He has called for a wave of lawsuits against governments and fossil fuel companies, telling The Guardian: “The judiciary is the branch of government in the US and other countries that is relatively free of bribery. And bribery is exactly what is going on.”
A significant number of legal actions have been launched, particularly in the US, to force governments to take action on climate change. Results have been mixed.
On September 23, climate activist Greta Thunberg and fifteen other young people filed a complaint via the United Nations. The case targets five major carbon polluting countries for violating the United Nations Convention on the Rights of the Child.
In a covering note regarding his expert report, Hansen writes: “Despite the recent turn toward increased global authoritarianism and denial of scientific facts, we have also recently witnessed the heart-warming sight of marching Australian children, defying their Prime Minister’s instruction to stay in school.
“It may not be long until there is another chance at a day of reckoning. This time it must be clearer what young people and other life on our planet need to assure their future.”