Talk by Joanna Haigh
There has been a marked acceleration in global warming since the mid-20th century with an average temperature increase of 1°C between 1961 and 2016 (measures against the pre-industrial period which serves as a reference). The effects are clearly visible, one of the most remarkable being the sea level rise as the thermal expansion of water due to this increased warming causes this water to take up more and more of the ocean’s surface. The melting of the ice caps also contributes to this sea level rise, and the gradual retreat of the summer Arctic sea ice is a further indication of global warming.
Our energy system is based on solar energy. The Earth absorbs heat and emits thermal radiation into the atmosphere. Half of solar radiation is captured by the surface of the Earth. Surface temperature therefore increases and produces heat radiation, of which 90% is absorbed into the atmosphere. The atmosphere then reheats and feeds back into the heat absorbed by the surface of the earth. But greenhouse gases trap thermal radiation close to the surface of the Earth, as the atmosphere is essentially open to solar radiation but not thermal radiation.
The heat-trapping ability of CO2
Steam (H2O) has the highest overall warming effect, but this is without the direct influence of humans who do, however, contribute to the carbon emissions (CO2) currently representing a third of those from steam. From a physics perspective, it is interesting to note that the spectrum of CO2 is such that absorption occurs even in the absence of steam. This gas therefore has a significant effect. CO2 is a powerful greenhouse gas. The measurements taken at various intervals show in particular that atmosphere’s CO2 quantity has increased dramatically over the last five decades, from 320 parts per million in 1960 to over 400 today (up 20%). Looking back, by thousands of years, we see that human life has never been exposed to such a high concentration of CO2 as it is today. The situation we are in today is unprecedented.
Alongside the changes in the atmosphere’s CO2 concentration, changes have been observed in temperatures over the various ice ages.
We even see a correlation between the two. It is this very fact that has led many to conclude that in light of the last few thousand years these changes are a natural occurrence linked to changes in the Earth’s orbit around the Sun and amplified by CO2 and CH4 (methane) emissions. In these conditions, temperature increases ten times faster which too is unusual.
The measurements taken at various intervals show in particular that atmosphere’s CO2 quantity has increased dramatically over the last five decades
Added to natural factors are human factors: industrial pollution, modification of terrain due to exploitation and certain farming practices. Computer modelling of accelerated warming between 1860 and 2000 gives a clearer picture by comparing a scenario accounting for natural factors (sun, volcanoes) with a scenario accounting for all factors, including greenhouse gas.
Yet global warming is not uniform and varies significantly across the globe. Looking at the planisphere established for the period 1901-2012, we see that global warming appears to affect first and foremost the continental regions, whereas there is almost no increase in temperature in the North Atlantic area. Climate experts are grappling to explain this phenomenon, which could be to do with the ocean’s internal circulation in this part of the world. Sharp temperature increases of more than two degrees have been observed as localised to continental Asia and South America.
The necessary development of alternative sources of energy
To predict what our future will look like we will need to think about CO2 and, by extension, the future of our economic models and the way we use energy. We can start by drawing up scenarios on CO2 concentration and emissions, keeping in mind that we cannot use the same time and quantity reasoning for these two different elements. Reducing emissions will not guarantee, in the short run, a return to an acceptable concentration level. As for warming forecasts, the results range from 1° to 5° in the space of a century. But one thing is sure: to stop global warming we will need to stop CO2 emissions, and not just make promises to reduce them. Only, it will be difficult to stop CO2 emissions altogether. We can hope that new technologies will enable us to capture and store CO2 in the atmosphere.
Scenarios on future temperatures and rainfall have also been drawn up. It is very difficult to obtain detailed models on expectant rainfall.
We already have an idea of what impact greenhouse gas emissions will have by 2100.
Existing models nevertheless suggest that certain regions will soon become wetter, while the Mediterranean and North African area will be far more arid. We already have an idea of what impact greenhouse gas emissions will have by 2100, and the news is not good: two billion people are expected to face water shortages; 10 to 12 billion people are expected to face heatwaves every year; and 70 to 90 million people are expected to be affected by flooding every year. All these scenarios spell disaster.
While there was much enthusiasm around the Paris Agreement and the principle of countries committing to reducing their gas emissions is encouraging, the COP 21 target falls short of the real objective. But we absolutely have to reach the 1.5° target. Developing recyclable and renewable energies has therefore become an imperative. To date, 21 countries have committed to doubling their investments in green energy.
On 1 June, US President Donald Trump announced the withdrawal of the United Stated from the Paris Agreement. He first spoke about renegotiating it, revealing a profound ignorance of the very principle of the Agreement which is founded on the commitment of the signing countries. Yet despite this he can decide not to apply it to the United States, which is worrying prospect. We are all concerned by this challenge which is also linked to consumption and individual behaviour.
Joanna has been Co-Director of the Grantham Institute at Imperial College since 2014. She studies radiative transfer in the atmosphere, climate modelling, radiative forcing of climate change and the influence of solar irradiance variability on climate. She is a Fellow of the Royal Society, the Institute of Physics, the City & Guilds and the Royal Meteorological Society and an Honorary Fellow of Somerville College Oxford.
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