Hasn’t the climate always changed, so why worry?

Hasn’t the climate always changed, so why worry? –  The world’s climate has always been changing, yet the speed of change and its direction have not always been the same. In the last 10,000 years, the climate has been remarkably stable, with average global temperature estimated to have ranged between -0.4 and + 0.4 degrees Celsius, relative to the average between 1961 and 1990. Carbon dioxide levels remained quite stable during this period, at around 280 parts per million, up to the recent industrial age, when they both shot up in unison (Figure 1).

The Role of Carbon Dioxide
Figure 1.

Hasn’t the climate always changed, so why worry?

We also have good records for temperature and carbon dioxide, up to 800,000 years old, from the gaseous composition of air trapped in ice cores (figure 1). Every three meters deeper you will find air that is about 300 years older. With ice cores to three kilometers deep, we can go far back in time.

Ice cores show that the remarkably stable warm period of the last 10,000 years is actually an exception. About 16,000 years, ago we were in a cold ‘glacial’ period, up to 10 degrees Celsius colder than today, with polar ice extending across North Europe and North America.102 Such glacial periods are historically the norm, as for about 80% of time in the last 800,000 years we experienced glacial periods with temperatures at least 4 and often 8 degrees Celsius colder. Only every 100,000 years or so did warmer periods of about 10,000 to 15,000 years takes place, at about the same temperature as in the twentieth century.

Ice core data provides the best evidence of the link between carbon dioxide and temperature, as they follow each other closely in this period, with a delay between initial warming and mass carbon dioxide release established at 200 years. This interaction and the cause of warm periods to glacial ages can be explained in six steps;

Hasn’t the climate always changed
Credit: BBC
  1. Warming starts because of changes in how the earth rotates on itself and spins around the sun (the orbital cycle). Every 110,000 years or so it is just in the right position for the northern part of the globe to get a lot more sunlight (northern Europe, Russia, Greenland, Arctic, Canada, northern United States).
  2. The warming melts large parts of the ice sheets that go up to Europe, Canada, and North-China, and releases warm freshwater (no salt), which changes the ocean currents that make northern Europe warm and results in a warming of the Southern Hemisphere and oceans.
  3. Now temperature feedback turns the oceans into a net emitter of carbon dioxide. Deeper ocean water flows upwards as the ocean heats up, which contains a lot of dissolved carbon dioxide and dead microorganisms composed of carbon. The carbon-rich water gets closer to the surface, a lot of fish grow which feed on the dead microorganisms that breathe out carbon dioxide like humans, and this happens at such mass rates that the ocean starts to release carbon dioxide instead of absorbing it.
  4. The large amounts of released carbon dioxide result in more heat being trapped which accelerates warming and turns ice ages into warmer periods. Carbon dioxide levels shoot up from 180 parts to 280 parts per million in the atmosphere in these transitions. Now the earth enters a warmer period as we have experienced in the last 10,000 years.
  5. As the earth’s orbit continues to change during the 10,000 to 15,000 or so warmer years, the Northern Hemisphere receives less sunlight and becomes gradually colder. Snow is more frequent which turns into ice, and as snow and ice reflect more sunlight than bare earth, it accelerates the process, leading to a colder climate.
  6. A glacial period starts when large amounts of added carbon dioxide from the earlier warmer release get absorbed again because of cooler ocean waters (dropping levels back to 180 parts per million). The glacial period only ends when the earth’s orbit is again just in the right position to let the sun melt the majority of the vast sheets of ice that have formed in the north (step 1 above).

The cycles of glacial and interglacial warm periods were regular in the last one million years, yet we can go further back—much further—thanks to crystals and seafloor ‘soil.’ Sediments composed of decaying shells and sea life, as well as geochemical compositions of crystals, allow for the reconstruction of temperature and carbon dioxide records up to 550 million years ago.

Deep time records are much fuzzier and only provide general trends at geological timescales. We can see that about 2.5 million years ago, the world started to experience an ice age, as the polar caps became permanently covered with ice, which kicked off the glacial-interglacial dynamics that we are still in today. The earth before the ice age was much warmer. It cooled, very slowly from a temperature of 12 degrees Celsius warmer than today, nearly 49 million years ago, about 17 million years after the dinosaurs became extinct (figure 1). It was so warm at that time that palm trees grew in polar regions. Proxy records of carbon dioxide up to 65 million years ago show a less distinct relation to temperature, partially still unexplained. Carbon dioxide levels rose slowly from around 400 to 1,600 parts per million from 50 to 65 million years ago and began a long descent that stopped at about 200 parts per million reached around 10 million years ago. 

A part of the explanation is the net uptake of carbon dioxide by plants in the ocean and land to produce oxygen. As plants decay, the carbon dioxide contained as carbon in their roots, stems, and leaves gets stored in soils and ocean sediments, and also results in the added formation of oil, gas, and coal. By slowly taking more and more carbon dioxide out of the atmosphere, the amount of ‘trapped’ heat declined, and the climate cooled down.

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