How the current changes in climate are so different to earlier changes
Back in October, Prof Brian Golding invited three eminent lecturers to present the evidence for climate change in "Our Fragile Earth": 'Our Fragile Earth', climate change events this autumn.
Here, Prof Golding discusses a few of the issues that have been raised about this evidence. On February 18th , he will be giving a follow-up Café Scientifique at 3pm in Kennaway Cellar, entitled "Our Fragile Earth: So what do we do" in which he will review the evidence and discuss ways in which individuals can make a difference:
Café Scientifique: Our Fragile Earth
First, a few basics. All of the steps that connect increasing Carbon Dioxide to increasing global surface temperature can be demonstrated, mostly under controlled laboratory conditions. Indeed the science was well understood back in the 19th century. They just didn't believe that Carbon Dioxide could possibly increase as much as it has.
The average temperature of the earth is 14oC. Without our blanket of greenhouse gases it would be -18oC and life would probably not exist.
Since the industrial revolution, the amount of Carbon Dioxide in the atmosphere has increased by almost a half. Indeed, there is now one third more Carbon Dioxide in the atmosphere than at any time in the last 800,000 years.
Basic laboratory physics shows that an atmosphere with more Carbon Dioxide will retain more of the sun's heat than one with less Carbon Dioxide.
Global temperatures have increased decade-on-decade since the mid-20th Century, consistent with the basic physics.
The warming is reinforced because a warmer atmosphere holds more water vapour, which also retains heat.
We are as sure as we can be that the observed warning is caused by the observed increase in Carbon Dioxide and that the increased Carbon Dioxide comes from the burning of fossil fuels. .
The longest direct record of earth's climate that we have was obtained from analysis of air bubbles in an ice core from the Vostok base in the Antarctic. The picture below shows the temperature (top, in red) and carbon dioxide content (bottom, in blue) of the air when the bubbles were trapped in the ice. You will see that ice ages (low temperature) and inter-glacials (high temperature) have very variable lengths, and while a new ice age may be expected in the next few thousand years, there is no reason to expect it soon. As shown by the extreme right hand end of the carbon dioxide graph, we are now in uncharted territory.
Critics point out that the carbon dioxide content changes after the temperature and not vice versa as one would expect. However, science attributes the ice ages to variations in the earth's orbit, not to changes in carbon dioxide. During cold periods, more carbon dioxide can be locked up in the deep oceans, causing the reduction in the atmosphere, and this may have the effect of"locking" the earth into the ice age. The last time CO2 was as high as now, was before the ice ages started, when the earth was much warmer than now (long before any humans were around!)
Ten years ago there was much speculation about a hiatus in the rate of rise of global temperature. As the graph below shows (see also https://www.youtube.com/watch?v=sHLHmnYuTR4) this was largely an illusion caused by the very high temperature in 1998, which resulted from a combination of the increasing trend with a naturally very hot year due to a strong El Nino. The next such combination occurred in 2016, when it became clear that there was no hiatus. The graph shows a steady climb in average global temperature since the 1970s. Prior to that was a period when volcanic eruptions held the temperature down for several decades..
Climate models, like weather prediction models, are based on the laws of physics that govern how the atmosphere will behave. By carrying out "what if" experiments, scientists can use the models to help their understanding of why climate is changing. The pictures below show reconstructions of global temperature (the green bands) compared with measured temperatures (red line) from 1850 to 2000. The only difference in the two graphs is that changes in carbon dioxide are omitted from the one on the bottom. Both contain recorded changes in solar radiation and the effects of volcanic eruptions. The top picture, in which the impact of carbon dioxide is included, matches the observed trends much more accurately, confirming that the processes represented in climate projections reflect the observed historical changes to a good approximation when the changing concentration of carbon dioxide is included. Note that the carbon dioxide effect (shown by the difference between the green band in the top and bottom pictures) begins to become significant from about 1960, when the post-war recovery really got underway, but that a natural reduction at that time due to volcanic eruptions compensated to keep temperatures down.
Comparison of simulations (green bands) with (top) and without (bottom) the observed change in carbon dioxide, compared with the observed change in global average temperature. The width of the green bands represents the natural variability of the atmosphere. (© Crown Copyright, Met Office, 2020).
The current global mean temperature has not yet exceeded the highest temperatures recorded in previous inter-glacials, but human society has developed in the current climate and the rate of change is unprecedented in human history. There is ample archaeological and historical evidence for failure of past civilisations due to periods of unusual climate. In our current situation such a failure would be on a global scale. The predicted higher frequencies of severe weather in a warmer climate are already being realised and, together with the effects of sea level rise due to the warming ocean, will increase weather-related threats across the world. Nevertheless, with the necessary investment in curtailing emissions and adapting to climate change, the people of the world may yet survive to a cleaner and healthier future.