Scientific view: the scale and the urgency (part 2)

In Part 2 of Dr. Emily Shuckburgh’s article, she highlights the increasing rise in CO2 levels from 1850 to the present day. Dr. Emily Shuckburgh is a climate scientist at the British Antarctic Survey, and is at the forefront of observing the dramatic and shocking changes in our ice sheets and greenhouse gas levels. Click here to read part one.

What is happening to our climate?

The world is warming, the climate is changing, and it is due to humans

That the world is warming and the climate is changing is beyond doubt. Records from thousands of weather stations across the world, and ocean data from ships and buoys, show the temperature measured at the Earth’s surface has increased substantially over the past century, and especially over the last fifty years. The global average temperature is now more than 1°C warmer than the pre-industrial era, Figure 1(a)

As the world has warmed, changes in many other features of our climate have also been observed. The oceans have been warming not only at the surface but also at depth. As the water warms it expands to fill a larger volume. This expansion, combined with input of water from melting ice from glaciers and the polar ice sheets, has led to a rise in global sea level of more than 20 cm since 1850, Figure 1(b). The extent of sea ice in the Arctic has undergone dramatic decline in recent decades. The change is so substantial that the area covered by sea ice at the end of the summer melt season is now about 2 million square kilometres less than at the end of the twentieth century – a difference equivalent to the combined area of the UK, Ireland, France, Spain, Germany and Italy.

Careful scientific assessment has concluded that the warming observed over the past 150 years is predominatly due to human activies. The recent special report of the Intergovernmental Panel on Climate Change, 1.5°C of warming, concluded that the estimated warming due to human activities matches the level of observed warming to within ±20%.

Human society has transformed over the past century and a half. There has been a six-fold increase in population. Back in 1850 the global population was around 1.25 billion. Today, both China and India have country populations in excess of this. At the same time, prosperity has increased enormously, with a hundred-fold increase in real terms in global GDP since 1850, Figure 1(c). While such economic measures may not be a particularly good indicator of societal progress defined in terms of the wellbeing of people and households, it is nevertheless clear that in broad terms significant societal progress has been made over this period in many countries of the world.

Much of the explosion in prosperity since the start of the industrial revolution has come about precisely because of that industrialization, the increase in which can be tracked by increases in energy use. Total global energy use has increased twenty-fold since 1850, incuding all domestic and industrial usage, Figure 1(d). This growth was accompanied by a shift from traditional energy sources – wood, wind and water – towards fossil fuels, first coal and then oil and natural gas. In 2017, fossil fuels made up almost 80% of the world’s energy use. Hydropower, wood, biofuels, and nuclear energy together accounted for just under 20%. New renewable energy sources, such as solar and wind, represented just over 3%, but their share is growing rapidly.

We have also transformed the land surface, cutting down forests to make way for settlements and farming. Both these activities and the burning fossil fuels release carbon dioxide into the atmosphere. The atmosphere forms a remarkably thin layer over the Earth; indeed, it is as thin in relative terms as the skin of an apple. Hence the dramatic increases in our global footprint have been associated with a change in the composition of our atmosphere. As a so-called “greenhouse gas” one would expect an increase in carbon dioxide in the atmosphere to be accompanied by an increase in temperature of the Earth’s surface and this is what has been observed.

Today’s atmosphere is unprecedented in human history, pre-history and beyond

The last time the Earth experienced broadly comparable levels of atmospheric carbon dioxide was during the mid-Pliocene, 3-5 million years ago. To find levels consistently above those of today you have to look much further back to the mid Miocene some 15 million years ago. To place this in context, the oldest object in the British Museum, a cutting tool made by early humans in Africa, dates from just 1.8 million years ago. Since the ability to create and use tools is the skill that sets us apart from other animals, this can be seen as marking the very start of the journey of humankind.

Hence, at no point have our forebears lived in a world with atmospheric carbon dioxide as high as it is today.

Putting the recent change into a longer historical perspective provides a sense of how unusual the current changes in climate are. One of the clearest pieces of evidence that tells us about that are the ice cores drilled from Antarctica. As the snow falls in Antarctica it traps with it air from the atmosphere. As the snow piles up layer-upon-layer, this air is trapped as bubbles in the ice. This means that as scientists drill down through the ice, more than 3 km deep, they are able to recover the ancient air that was in the atmosphere hundreds of thousands of years ago. The air bubbles can be analyzed to determine the carbon dioxide levels and the water in the ice can be analyzed to determine the ratio of different isotopes of oxygen, which gives an indication of the temperature in the past.

The longest ice core record we currently have is 800,000 years old (a European project is currently aiming to drill a core going back 1.5 million years). The world experienced a range of very different climate states over this period. During the last ice age, which peaked about 22,000 years ago, sea levels were some 130 m below where they are today. By comparison, during the last interglacial warm period, about 125,000 years ago, sea levels were probably somewhere in the range 5 to 10 m higher than they are today.

The ice cores show that over the past 800,000 years, carbon dioxide levels have varied between a low of about 180 parts per million (ppm) during the ice ages to a high of about 280 ppm during the interglacial periods. Today’s carbon dioxide levels of over 405 ppm vastly exceed this, clearly demonstrating that the current change lies far outside the natural cycle.

This article was originally published as one piece and has been kindly reproduced on the POW UK blog in three parts with Dr. Emily Shuckburgh’s permission. Part three, the final section, will be online soon, discussing extreme weather and the challenge of responding to this information so as to limit warming to the less catastrophic 1.5 degrees celsius, as recommended in the Paris agreement.