Consequences of a weakening gulf stream

The warm gulf stream is a current that brings warm water from the tropics to the poles, where it cools, sinks and returns southwards. It weakens because 1) warming makes water less dense and more buoyant and 2) fresh and cold water from the melting Greenland ice sheet disrupts the flow.

The warm gulf stream has weakened about 15% since 1950. Past collapses of the gulf stream have caused western Europe to descend into freezing winters. A significantly weakened system is associated with more – maybe much more – severe storms in Europe, faster sea level rise on the east coast of the US, increasing drought in the Sahel and collapsing deap sea ecosystems.

Figure. AMOC = gulf stream. Nature 556, pp180-181 (2018).

The system is said to be highly non-linear and has been associated with abrupt changes in temperature when disturbed, such as winter temperatures changing up to 10C within three years in some places.

Counterintuitively, the cooler water close to western Europe that is entailed in a weaker gulf stream helps warm air to flood into Europe from the south, thereby also possibly increasing summer heatwaves.

Mid-Pliocene conditions start in 2030

Under the Business-as-usual scenario (RCP 8.5) the future climates in 2030  will most closely resemble Mid-Pliocene climates, with surface temperatures 1.8 °C to 3.6 °C warmer than preindustrial temperatures. This means a climate that resembles the climate 3-3.3 million years ago, when CO2 -levels were at 400ppm – levels which we have crossed in 2016. Mid-Pliocene conditions will first emerge in continental interiors and in the decades afterward they will spread towards the coasts.

Figure: Temperature trends for the past 65 Ma and potential geohistorical analogs for future climates. By Burke et al. in Proceedings of the National Academy of Sciences (November 2018).

In 2100 the situation will be totally different. Under the RCP8.5 scenario, the climate from the past that best matches continental interiors by 2100 is the early Eocene climate. For these climate conditions, we have to go back abut 50 million years, when global mean annual surface temperatures were a whopping 13 °C ± 2.6 °C higher than preindustrial temperatures and there were swampy forests in the arctic.

Under the RCP4.5 emission scenario the climate stabilizes at mid Pliocene-like conditions.

Figure. 2050 analogies with earlier climates (RCP8.5)

Water resources worldwide are dwindling & rivers are drying up

Researchers at the University of South Wales, using data from 43,000 rainfall stations and 5,300 river monitoring sites across 160 countries, have found (2018) that water supplies worldwide are decreasing. Despite growing precipitation due to climate change, higher evaporation rates and dryer grounds that soak up more water leads to less water in rivers.

This study is in line with a study from 2009, that points out that world’s major rivers, amongst which the Yellow river in Northern China and the Ganges in India, are ‘drying up’. Causes are human activities such as dam building and diverting water for agriculture (that caused e.g. the Aral Sea to disappear) and climate change altering rainfall patterns and increasing evaporation rates. The only exceptions are rivers in the Arctic, the Brahmaputra in South Asia and the Yangtze in China. Very much likely because of additional melting of glaciers and ice sheets.

Experts predict that water levels of Euphrate and Tigris might drop 30-70% compared to end 20th century and speak about the “Fertile Crescent” disappearing altogether. Even notoriously wet places such as the United Kingdom are expecting severe water shortages by 2050s as summers get drier. The Colorado river has no longer enough water to go around.

Most of northern china suffers from acute water shortage (multicausal). In the past 25 years, 28,000 rivers and waterways have disappeared across the country. The iconic Yellow River, the second-longest in Asia, is now a tenth of what it was in the 1940s, and often fails to reach the sea. Former Chinese premier Wen Jiabao has warned the lack of water threatened the very survival of the Chinese nation itself.

200 million to 1.4 billion climate migrants by mid-century

How many climate migrants will there be around the year 2050? We did some research:

The world bank estimates 140 million internal migrants in Sub-Saharan Africa, Latin America and South Asia by 2050.

A United Nations report of 2009 estimated the number of people to be displaced internally in 2045 by desertification alone around 135 million

The UN International Organization for Migration forecasts 200 million internal and international (permanent and temporary) environmental migrants by 2050.

A paper from June 2017 (by Geisler et al) mentions 1.4 billion people that could be climate refugees by 2060 due to rising ocean levels (alone!).  A number that societies might not be able to support: “The colliding forces of human fertility, submerging coastal zones, residential retreat, and impediments to inland resettlement is a huge problem. We offer preliminary estimates of the lands unlikely to support new waves of climate refugees due to the residues of war, exhausted natural resources, declining net primary productivity, desertification [and] urban sprawl…”

12 million hectares of productive land become barren every year due to desertification and drought (alone), while food production needs to increase by 70% in 2050 to feed the entire world population. Drought, water scarcity and destruction of natural resources because of climate change (and overpopulation, etc.) might lead to conflict. The UNCCD reminds that 40 per cent of all intrastate conflicts in the past 60 years are linked to the control and allocation of natural resources.

Figure from Wikipedia, by KVDP

And for those that like smaller numbers: In 2017 there were (already) 24.2 million new displacements because of disasters.

And for comparison of the above numbers: In 2016 international migration numbered around 260 million people.

Weakening jet stream causes colder winters

In addition to a weakening gulf stream, another factor that contributes to colder winters (and probably warmer summers) in the northeastern United States, Europe, and especially Asia, is a weakening jet stream. The jet stream is a wind of about 180 km/h on average and up to 400 km/h in instances, that streaks around the north pole at 8km altitude. It gets its power from temperature differences between south and north. It is weakening because the arctic is warming fast. Warming in the arctic means less sea ice, which means more – dark – ocean that in turn absorbs more heat (i.e. a positive feedback loop).

A weaker jet stream meanders more, bringing warmer air to pockets in the arctic and colder air to pockets south. These ‘loops’ in the jet stream can get stuck, bringing about extreme events of cold spells and probably also heat and drought. Based on changes in the jet stream researcher predicted worsening drought in California back in 2004. The weaker jet stream does not mean that cold spells get colder though, cold extremes are getting less cold.

Figure: The Guardian.

Mid-Pliocene Climate: 2-3 °C warmer

3-3.3 million years ago global average temperature was 2-3 °C higher than today. The sea level was 25 meters higher and there was hardly any ice sheet in the northern hemisphere. Just as in predictive models, in the paleaontological data you also see a larger difference with today’s temperatures in the northern hemisphere than in tropical regions and southern hemisphere.

                  Pliocene temperatures (by Giorgiogp2)

Carbon dioxide concentration during the mid-Pliocene has been estimated at around 400 ppm (yes, the 400 ppm that we have crossed in 2016 for the first time in 3 million years).

The climate was not only warmer, but also wetter, resulting in a northward shift of the taiga (boreal forest) and tundra and a spread of tropical savannahs and woodland in Africa at the expense of deserts (am I reading this correctly: less desert?!)

                                                                           Biomes in mid-Pliocene

Best to live in U.S. above 42nd parallel

The Guardian dedicates an article to where you can best live in the U.S. when climate change gets serious. Broadly speaking you can best move to a band roughly above the 42nd parallel.

Places close to a reliable source of water without being flood-prone score high. Seattle, Buffalo, New York (with flood defenses), and Duluth, Minnesota are mentioned as places you might want to be at the end of the 21st century.  Places that score low are the south-west and south-east because they will be baked. Southern Florida will be submerged and the Gulf coast will be battered by hurricanes.

Extremely hot and dry conditions have doubled worldwide

Researchers show in Science Advances that the probability of extremely warm and extremely dry conditions has doubled worldwide.

They warn for the threat this poses to worldwide agricultural production. Before anthropogenic climate change chances of extreme weather in multiple agricultural key regions were slim. At the same time the global marketplace could compensate for a bad harvest in one region. The researchers see erosion of this buffer. They specifically point out that the frequency of these simultaneous extreme hot and dry conditions will increase with 20% in the next three decades.

Non-survivable humid heat to effect tens of millions in South Asia

If climate change is unchecked (RCP 8.5) South Asia  will experience heat waves that “exceed the survivability threshold” (sic!) from 2070 onwards. Researchers at MIT conclude in a 2017 study that 4% of the South Asia-population, about 70 million people, would face heatwaves of humid heat that will kill even healthy people in the shade within hours. The wet bulb temperature (WTB) would reach 35°C, which means that the human body cannot cool itself and shuts down.

Three quarters (1.3 billion people) in South Asia would experience at least one heatwave before the end of the century with wet bulb temperatures of 31°C, conditions of ‘extreme danger’. To compare: In 2015 there was a severe episode in wich 30°C WTB was measured. This led to 3,500 deaths in South Asia. According to this article the largest hospital in Karachi was receiving 1 patient per minute and the morgue was overflowing.

Wet bulb temperature. The figure most right shows non-survivable temperatures in heat waves under business as usual scenario (no carbon cuts, 2071-2100) at locations in the Chota Nagpur platearu, northeastern India, and Bangladesh. WTB temperatures approaching 35C would occur over most of South Asia (here: Pakistan, Nepal, India, Bangladesh and Sri Lanka), including the Ganges river valley, northeastern India, the eastern coast of India and the Indus Valley of Pakistan.

Wet bulb temperatures currently rarely exceeds 31°C. 35°C WTB has never been measured yet.

Persian gulf might become uninhabitable this century

Researchers of the Massuchusetts Institute of Technology (MIT) warn that if greenhouse gas emissions will not be curbed, large parts of the Persian Gulf could become uninhabitable at the end of this century. Once every decade heat waves might occur  in which the combination of temperature and humidity (the so-called “wet bulb temperature”) will exceed the equivalent of 35 degrees Celsius at 100% humidity. If such conditions are sustained for several hours they become fatal to the human body.

Extreme conditions such as these may lead to conflict and climate migration. Another study early 2018 points out that drought in Syria gave rise to more than one million domestic emigrants and how subsequently hopelessness in cities gave rise to an escalation of violence.

In the figure below two scenario’s of the IPPC, under which temperature in some areas of the Persian Gulf can reach 60 degrees Celsius at the end of the 21st century.