I have just finished the second week of learning with this free online course by the University of Exeter which I found on futurelearn.com. This post is supposed to help me summarise and remember what I have learned and to ask some questions or make comments about it. For everyone who has already finished this part of the online course, I would love it if you could read this post and tell me if you think I got anything wrong. If you only want to read my comments/questions, they are in bold. Thank you for visiting my website, I hope you enjoy! 🙂

What was covered?

This week was packed with lots of interesting information. To put it in a nutshell, I learned about ancient and recent past as well as contemporary climate change, the differences and similarities between them, the natural and anthropogenic forcing factors driving it and the key indicators that show our climate is changing very quickly nowadays.

Ancient past climate change

At the beginning of the week, I was taught that the Earth´s climate has self-regulated since the planet´s formation 4.5 billion years ago. Even though the sun was 25 to 30 percent less bright shortly after the birth of the solar system, there is evidence that Earth was as warm, if not warmer than today. With the blanket of warming gases we have in our atmsphere today, the Earth would have been frozen back then. Thus, there must have been a much higher atmospheric concentration of greenhouse gases present, especially of carbon dioxide and water vapour.

Chemical weathering
(source: online course)

Now the question is how all of that carbon dioxide was removed from the atmosphere as the sun became brighter. The online course explains that this was possible because of a process called chemical weathering. Chemical weathering: carbon dioxide from the atmosphere forms weak carbonic acid in rainwater -> dissolves silicate rocks when it reaches land -> washes into oceans as bicarbonate ions-> organisms use carbon for their shells -> formation of carbonate rocks from shells deposited at the bottom of the ocean (carbon dioxide from the atmosphere is transferred through the hydrosphere to be stored in the lithosphere!)

Moreover, the chemical weathering process is part of a negative feedback mechanism also involving temperature and atmospheric carbon dioxide concentration: higher temperatures -> more chemical weathering -> lower atmospheric concentration of carbon dioxide -> lower temperatures -> less chemical weathering…

At this point, I did not quite understand how temperature and chemical weathering are linked. However, a fellow learner on the course suggested that higher temperatures mean that more water evaporates, which causes more rainfall and thus more weak carbonic acid coming down with it.

Another riddle next to that about how atmospheric carbon dioxide levels were reduced as the sun became brighter is that about why the Earth froze over 2.2 billion and 700 million years ago and how this was reversed again. I learned that the reason for the occurrence of these two events is the positive ice albedo feedback triggered by the growth of continents and the subsequent higher rate of chemical weathering: growth of continents -> more chemical weathering (more rocks for carbonic acid to react with) -> lower atmospheric concentration of carbon dioxide to warm planet -> cooler temperatures -> more ice -> higher albedo -> even cooler -> more ice…

Carbon dioxide trapped in atmosphere (source: online course)

When the cooling is sufficient for the ice cover to reach the tropics, there is a tipping point and Earth freezes over completely. So how was this reversed? Because the planet was covered in ice, chemical weathering could not take place. However, volcanoes kept pumping carbon dioxide into the atmosphere and it started building up as it had nowhere to escape. Consequently, more and more heat was trapped by this thick blanket of carbon dioxide and the ice at the equator melted. The following positive feedback (less ice -> lower albedo -> hotter -> less ice) runs away in the other direction and brings about a superheated planet without ice caps if the climate is not able to stabilise itself in time.

This part of the online course taught me a very important lesson. We often think of the Earth´s systems as being extremely good at remaining in balance and maintaning the climate which supports life on the planet. Consequently, many people do not believe that our actions, like the burning of fossil fuels, could really have a significant impact on the Earth´s climate. Looking at the past and realising that the climate has key instabilities and feedback mechanisms that can really destabalise the entire system once they are triggered , can help us to realise how important it is to change our actions in order to ensure that we do not end up with a superheated or frozen planet anytime soon.

Earth orbits the sun in habitable zone (source: online course)

Furthermore, the online course talked about how the Earth orbits the sun in the habitable zone, where water occurs as a liquid. As the sun will get brighter, this habitable zone will move further outwards and Earth will come ever closer to the inner border of this zone. This will cause rising temperatures and more water to evaporate. The water vapour in the atmosphere will become very effective at trapping more heat and eventually, the oceans will have evaporated completely and our planet will see the end of the biosphere. Luckily, the sun is only getting brighter very slowly.

Thawing of methane hydrates (theoldspeakjournal.wordpress.com)

Last in this section about ancient past climate change, I found out about the Paleocene-Eocene Thermal Maximum (PETM), which describes a rapid jump in global temperature about 55 million years ago. Typically, changes of 100s of parts per million in the atmospheric concentration of carbon dioxide take place over the course of hundreds of years, which allows life to adapt to the changes in climate. The PETM saw a 5 degree celsius increase in global temperatures over the course of 20,000 years. Oceans acidified and warmed by 6 degrees celsius. The warming was caused by a rapid release of greenhouse gases likely due to volcanism triggering a number of destabilising positive feedback mechanisms. Moreover, the initial rise in temperatures led to the thawing of methane hydrates, because of which large amounts of methane were released into the atmosphere, where they caused even more warming.

Recovery from the PETM was possible largely because of chemical weathering, which removed carbon dioxide from the atmosphere and helped neutralise the oceans. I was initially not sure why chemical weathering would neutralise oceans, because I had only learned about carbonic acid reacting with silicate rocks and the product being washed into the oceans. I thought it should acidify them further, if only slightly. However, I researched a bit and it seems that the bicarbonate ions, which result from the reaction of carbonic acid with the rocks, have an alkalizing effect in the water.

The PETM had similar effects on life on Earth as contemporary climate change, for example extinctions, sea-level rise, movement of ecosystems, destruction of coral reefs etc. However, during this event, our planet experienced warming of about 0.025 degrees celsius per 100 years. Nowadays, warming is on track for a 1 degree celsius increase per 100 years. It took the climate 100,000 years to recover from warming during the PETM. How long would it take the Earth to recover from contemporary climate change?

I have to say that I thought learning about the PETM was one of the most interesting, if not the most interesting part of the week. It was an unprecedented event which saw an abnormally rapid increase in global temperatures. Comparing this event, which had horrible effects on ecosystems and from which recovery took 100,000 years, to contemporary climate change really is a massive warning to us. Warming nowadays is ten times more rapid and recovery could take hundreds of thousands of years, especially if we tip the methane hydrate reservoirs into an irreversible positive feedback. We should definitely learn from past events like this, which show us that there is really no natural, non-human related explanation for the changes in climate we experience today.

Recent past climate change

Milankovitch cycles: eccentricity (source: online course)
Milankovitch cycles: obliquity and precession (source: online course)

At the start of learning about more recent past climate change, I was told that climate changes on time scales of thousands of years are highly predictable because they are connected to the way the Earth moves around the sun. Milutin Milankovitch described how the shape of the Earth´s orbit around the sun (circular or elliptical), the tilt of the Earth´s axis (between 21.4 & 24.5 degrees) and the wobbling of the Earth´s axis (like a spinning top) determine how much energy different parts of the Earth receive from the sun throughout the year and thus what the climate is like.

However, the course explained that the causes for climate changes on shorter time scales are much more unpredictable. Thus, evidence is needed to asses the impact of different influences. This evidence can come from tree rings, ice cores, pollen, ocean and lake sediments, or corals.

Tree rings: Width of ring -> rate of photosynthesis that year-> amount of sunshine during summer of that year -> temperature during summer (need of cross-dating with other trees and their ancestors -> wood from floor of forests/lakes/peat bogs)

Ice cores: carry composition of ancient atmosphere trapped in tiny bubbles -> ratio of deuterium (2H) to hydrogen (1H) & of oxygen isotopes (18O : 16O -> have different weight); 18O harder to evaporate & preferentially condensed out of atmosphere before air reaches poles; colder climate: these effects are amplified -> ice even more depleted in 18O (and in deuterium)

Different pollen for different plant species (source: online course)

Pollen: uniquely shaped to specific species; each plant has very specific needs -> soil acidity, atmospheric carbon dioxide concentrations & temperatures, precipitation levels… -> analysing pollen grain from layers of soil to reconstruct past climate

Ocean & lake sediments: 1)Ocean: ratio 18O : 16O in carbonate shells of marine organisms (colder climate -> more ice, less water evaporating -> even less evaporation of 18O than otherwise -> ocean more enriched in 18O); 2)Lake: layers like tree rings -> thickness of layers & 18O : 16O ratio -> information about past climate, e.g. amount of meltwater entering glacial lake each year, temperature, sources of precipitation, evaporation from lake

I learned that climate data from the sources listed above can be matched to events and cycles related to natural forcing factors like volcanism (It was interesting to learn that volcanic eruptions cause cooling, at least short-term, because the ash clouds block out solar radiation). However, these natural factors fail to explain the changes in climate our planet has experienced over the last 100 to 150 years. When comparing a graph showing the increase in global temperature over that period to a graph showing atmospheric carbon dioxide concentrations, one can see that there is an obvious connection between the two and conclude that carbon dioxide is the major cause for warming today.

Indicators for contemporary climate change

Lastly, I found out about the fact that climate data is collected very widely and reliably nowadays thanks to new technology and that this huge amount of data allows scientist to see the signs for contemporary climate change very clearly. The online course introduced me to the following key indicators:

Global mean temperature over land (source: online course)

Global mean temperature over land & ocean: most recent decade warmest on record; Earth now 1 degree celsius warmer than in pre-industrial times; warming not the same everywhere: higher latitude areas (e.g. Arctic) experience more warming & land warms faster than oceans; intervals of slower/faster warming (e.g. slowdown after 1997/8 El Ninõ event, but after slowdown: temperatures rising at accelerated rate)

Arctic sea ice area
(source: online course)

Sea ice extent: temperature rises are highest in Arctic (roughly twice as fast as global average) -> since 1980: surface of Arctic sea ice in summer fallen from about 4 million km2 to about 1.5 million km2; especially concerning because of ice albedo feedback; global sea ice extent 2016 lowest on record

Frequency of extreme weather events: warmer atmosphere holds more moisture; data hints global averaged precipitation may have increased over last century, is expected to increase in the future; precipitation unevenly distributed -> contrast between wet & dry regions will increase; uncertainty for future of ecosystems & societies, especially with growing population

Sea level (source: online course)

Sea-level: since 1880: steady rise in sea levels; about half of the rise due to thermal expansion, half due to melting of land ice; melting sea ice does not contribute to sea level rise -> floating ice displaces same volume of water as if it were liquid.

At this point, I had one last question, this time about the situation in Antarctica. During the online course, it was said that it is much colder in Antarctica than in the Arctic, that the ice there is not melting and that the area is thus not contributing to sea level rise. However, in an article on the NASA website, it says that “Antarctica lost about 127 billion tons of ice per year during the same time period (1993-2016)”. Maybe scientists just are not completely sure about what is going on there yet?

Conclusion

The message I take from this past week of learning with the online course is that evidence from sources like tree rings, ice cores or weather records as well as the comparison between contemporary and past climate change shows that natural forcing factors are not the explanation for the changes we see today. The difference between the PETM and contemporary climate change is not only that change is happening much faster this time, but also that humans are causing it and that we are aware of our responsibility as well as of the solutions to the problems we have caused.

If we do not want what happened during the PETM to happen again, we really have to change something about the way we live and about the primary goals our societies and economies are after. They should not be profit and power because that harms our environment as well as a large portion of the human population. Each of us can make a decision on how they want to impact the environment and other people, and that decision is important. Because remember: You may be a small part of this world, but you are not insignificant!