With a warmer climate, more energy will be involved in driving the weather systems. This is reflected in stronger winds [L125] and increased evaporation which in turn gives more precipitation. But the precipitation is not evenly distributed. Areas that previously received a lot of rainfall will get even more, while dry areas can get even drier. This has effects on biodiversity. Some species fail to adapt and migrate to other places or are significantly weakened. We also have a greater risk of humid heat waves, which will be a major health challenge in many places [L126].
The figure is based on running 26 different climate models from the CMIP5 project (see Climate models). The figure shows how the temperature increase (top row) has been distributed over the planet. The first column shows the effects of 1.5 °C warming since pre-industrial times. The middle column shows the effects of 2.0 °C warming. The last column shows the difference between the previous two columns. In the bottom row, corresponding figures show the percentual change in precipitation since pre-industrial times.
Neither change in temperature nor precipitation is evenly distributed across the planet. In some areas, the warming is significantly higher than the global average of 1.5°C. Extreme temperatures also become unevenly distributed geographically, and unevenly distributed throughout the seasons. For example, the winter temperature in northern latitudes could be 4.5 °C warmer than today, at conditions with abnormal heat ([L38], Chapter 3). Similarly, extreme summer temperatures further south could be 3.0 °C warmer than today.
Precipitation is also unevenly distributed; more precipitation in the north, while the Mediterranean and areas at a similar latitude get less rainfall. Combined with extreme warm periods in the same areas, this will make it quite unpleasant for the inhabitants. The frequency of forest fires will also increase.
These changes apply to a global warming where the average warming is 1.5 °C. As the figure above shows, these effects are amplified if the heating reaches 2.0 °C. The lack of willingness to act among world leaders, and the uninterrupted increase in greenhouse gas emissions (see Greenhouse gases), makes it unlikely that the heating will stop at 1.5 °C (or 2.0 °C, for that matter).
A good overview of climate change in Norway can be found on the website of the Norwegian Centre for Climate Services (NCCS) [L41]. The biggest problem for Norway is increased rainfall (18% increase in 2100 if greenhouse gas emissions continue to increase at the same rate as today). Episodes with short-term intense rainfall will also increase, both in number and intensity. In addition, sea-level rise will be a problem, but this is partially compensated for by the ongoing land uplift after the last Ice Age (see below about sea level). The land uplift is unevenly distributed along the coast and is estimated to be within 15 to 55 cm in 2100. The Norwegian Environment Agency has published a report on sea-level rise in Norway: [L42].
Two mechanisms cause the sea level to rise during global warming; warmer water in the ocean requires more space, and melting of ice from Greenland, Antarctica and glaciers around the world. According to the IPCC, warmer seas have led to an increase in sea level of approximately 0.8 mm per year in the period 1971-2010. In the same period, melting from glaciers has been responsible for a 0.25-0.99 mm increase in sea level per year ([L49] page 47). The following figure, taken from [L49], shows the observed sea-level rise (the colours represent different observation methods) until about 2015, and the simulated sea-level rise until 2100. The simulation is based on two emission scenarios: RCP2.6 blue curve and RCP8.5 red curve. RPC stands for Representative Concentration Pathway [L50] and is used by the IPCC to stipulate emissions from now to the end of this century. RPC2.6 is the most optimistic of these scenarios where emissions reach a maximum between 2030 and 2050, and then slowly decline to around current levels in 2100. RPC8.5 is the most pessimistic scenario where emissions continue to rise to about three times the current level.
An article in Nature from August 2020 [L111] provides a more recent analysis of the sea-level rise. A summary of this article can be found at [L112] (the Nature article must be paid for). The article shows that sea-level rise more or less stopped in the 1955-1970 due to the construction of dams (for example, the Aswan Dam in Egypt) which prevented the fresh water from reaching the sea. After 1970, sea-level rise picked up speed again, and has during the period been around 3.3 mm per year in 1993-2018.
In the long run, melting of the Greenland ice sheet and the ice in Antarctica will make a far greater contribution to sea-level rise than is shown in the figure above [L29]. But this will take time. In 10,000 years, the sea level will have risen between 25 and 52 metres, depending on how much CO₂ we release altogether. This is a rather reliable prediction because our emissions cannot be reversed. A significant portion of the CO₂ emitted now will remain in the atmosphere for thousands of years and contribute to the greenhouse effect, keeping the temperature 2-5 °C above pre-industrial levels. Even with 2 °C heating, the Greenland ice sheet and large parts of Antarctica will melt completely, but slowly. Most of the melting will take place during the first 2000 years, and in approximately 500 years, the melting will cause 2-4 metres of sea-level rise per century.
Sea-level rise is unevenly distributed across the world. Our region (Scandinavia, parts of Canada and Alaska) will be less affected because of land uplift that has been taking place since the last Ice Age [L51]. However, a few parts of this region are still sinking. This is shown by satellite measurements that measure the distance down to the earth's surface with an accuracy of a millimetre (see [L96]). For example, Bjørvika in Oslo is sinking more than 1 cm per year [L97].
Many studies have examined the frequency and intensity of tropical cyclones, such as Katrina [L117] in 2005, under global warming. An article from 2020 published by The American Meteorological Society [L118] summarises the conclusions from recent research. The article deals with a number of model studies that examine the consequences of a 2 °C warming above pre-industrial levels. It seems that the frequency of such cyclones actually will decrease, but the cyclones that occur will be more powerful. Several of them will reach a strength of 4-5, and the amount of precipitation that the cyclones emit will increase by around 14%. Combined with higher sea levels this will cause serious damage in coastal areas where the cyclones hit land. Cyclone Harvey, which hit Texas and Louisiana in 2017, caused damage amounting to $90 billion. 30-72 billion of this may be attributed to increased rainfall due to global warming [L57]. More about this cyclone can be found in Economy.
More extreme weather, drought and changing climatic conditions locally pose major challenges for agriculture around the world. Admittedly, more CO₂ in the atmosphere stimulates growth, but the negative consequences of global warming are dominant. An OECD article [L119] claims that population growth, increased demand and limited supply of agricultural land will result in increased real prices in 2050 of important agricultural products, such as rice and wheat (an increase of 25%), and maize (an increase of 50%). These estimates are made without taking into account the additional problems caused by global warming. With the most extreme scenarios for global warming, prices could rise by a further 30%. Admittedly, there is great potential for the introduction of better agricultural practices. The article Climate change adaptation in agriculture: practices and technologies [L120] published by the Consultative Group on International Agricultural Research (CGIAR) presents a number of measures for adapting agriculture to changing climatic conditions around the world. In the long run, maybe the introduction of vertical farming (see [L121] and [L82]) could supplement traditional forms of agriculture.
The rise in temperature will affect other processes on the planet, which in turn may affect the global temperature. Such feedback can either contribute to a strengthened increase in temperature (positive feedback), or have a cooling effect (negative feedback). Wikipedia has a comprehensive article on such feedback mechanisms [L47].
Many of the feedback mechanisms have been taken into account in the climate models used by the IPCC to estimate future global warming. This applies, for example, to the increase in water vapour in the atmosphere due to a warmer atmosphere. Since water vapour is a powerful greenhouse gas, this increase will lead to further heating [L48]. The impact of less sea ice in the Arctic (see Arctic animation) leading to more solar radiation absorbed in the sea, is also taken into account in the models. Other mechanisms are not so well represented in the models because the knowledge base is more uncertain. This applies, for example, to emissions of methane from permafrost that thaws in the Arctic. The potential effects of such processes can be large, and such processes could easily be triggered as global warming grows in strength. The warming will also increase the frequency of forest fires which will intensify CO₂ emissions.
Such feedback mechanisms can initiate a process in which one mechanism increases the heating so much that other mechanisms begin to operate. For example, the melting of sea ice in the Arctic and less land covered by snow causes more sunlight to be absorbed instead of being reflected back towards space. This in turn could increase greenhouse gas emissions from melting permafrost. This may result in the warming being able to continue on its own accord, even though we eventually manage to limit emissions of greenhouse gases. Two articles discuss the possibility of such scenarios.
The first article is taken from Nature [L113], Climate tipping points - too risky to bet against. It investigates various scenarios where the climate system reaches a tipping point that cannot be reversed. Some of these tipping points apply to the land ice in Greenland and two areas in Antarctica. The article assumes that we have already reached the tipping point where melting of these areas cannot be reversed. But how fast it goes depends on how large the heating will be. With 1.5°C warming, this could take 10,000 years, but if the warming reaches 2.0°C or above, the melting will only take around 1000 years. Such a meltdown will result in a 10 metre rise in the sea level.
Another such tipping point will destroy most of the planet's coral reefs at a warming above 2 °C. Such destruction is already visible off the coast of Australia (The Great Barrier Reef [L114]).
Deforestation and climate change are endangering the Amazon rainforest, and large areas of coniferous forest further north, in Canada, Alaska, Scandinavia and Russia are also at risk. These forests store large amounts of carbon which can escape into the atmosphere if the environment changes [L115]. Permafrost in the northern areas is at risk of melting, thus emitting large amounts of CO₂ and CH₄. These processes (deforestation and melting of permafrost) could potentially emit 300 gigatonnes of CO₂, and contribute to further warming.
The second article, Trajectories of the Earth System in the Anthropocene [L116], discusses possible scenarios for the evolution of the planet. The authors call the most likely scenario the "Hothouse Earth". The planet there ends up in a relatively stable state, with a significantly higher temperature than now. The article gives little specific information on how high the temperature will be. Nor does it suggest how long it will take before we reach such a state. An alternative scenario is called "Stabilized Earth". which is a stable state with temperatures 1-2 °C above pre-industrial levels. Getting there requires major changes, first and foremost cuts in greenhouse gas emissions, but also ensure that the natural systems that take up and stores CO₂ is taken care of and preferably reinforced. Capture of CO₂ from the atmosphere is also mentioned.
There are also negative feedback mechanisms, which have a cooling effect. For example, more heat radiates into space as the planet warms. Most of these mechanisms are, however, built into the climate models.
Latest update: 2021-07-20