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.
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.
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.
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.