 Figure 1: The Amazon rainforest to reach irreversible tipping point. Image: Al’fred / Shutterstock.com Loading the atmosphere with five million tonnes of CO2 every hour has pushed the Earth dangerously close to a no-return threshold, beyond which lies an inhabitable hothouse world. There are 15 known tipping points in the planet's complex climate system, and nine of them-including permafrost, the Amazon rainforest, the Greenland ice sheet, Arctic sea ice, and the Atlantic Ocean's circulation - are drifting alarmingly, as reported in the journal Nature. The authors of this journal persist that it is an existential threat to civilization, implying that the Earth is in a state of planetary emergency. Tipping points are reached when particular impacts of global heating become unstoppable, such as the runaway loss of ice sheets or forests. In the past, extreme heating of 5 degC was thought necessary to pass tipping points, but the latest evidence suggests that this could happen between 1 degC and 2 degC.The planet has already heated by 1 degC and the temperature is expected to rise further, due to past emissions and increasing greenhouse gases. The authors further warned that one tipping point, such as the release of methane from thawing permafrost, may fuel others, leading to a cascade. However, the researchers who wrote in a commentary article in the journal Nature acknowledged that the complex science of tipping points means great uncertainty remains. But they say the potential damage from the tipping points is so big and the time to act so short, that to misapprehend this situation is not a responsible option. They call for urgent international action. Prof Tim Lenton at the University of Exeter, the lead author of the article, said that the threshold for a cascade of interrelated tipping points has already been crossed. Phil Williamson at the University of East Anglia, who did not contribute to the article, also agreed on the prognosis made by Tim Lenton and his colleagues and said that we might have already lost control of the Earth’s climate. This new article comes as the United Nation warns action is very far from stopping global temperature rise, with the world currently on track for 3-4 degC. The commentary lists nine tipping points that may have been activated as shown in Figure 2.  Figure 2: Schematic diagram illustrating nine tipping points that may have been activated.
All signals indicate that part of the west Antarctic ice sheet may be in irreversible retreat as well as the Wilkes basin in east Antarctica. The collapse of these ice sheets would eventually raise sea level by many meters. The massive Greenland ice sheet was melting at an accelerating rate while the Arctic sea ice is shrinking fast. Alarming evidence indicates that the Permafrost across the Arctic is beginning to irreversibly thaw and release carbon dioxide and methane. According to a few studies, the Gulf Stream current in the Atlantic which brings heat to Europe has also slowed down by 15% since the mid-20th century. The scientists report that 17% of the Amazon rainforest has been lost since 1970 and the tipping point, where loss of forest leads to it drying out, could lie in the range 20%-40%. In temperate forests, especially in North America, heating has triggered more fires and pest outbreaks, potentially turning some regions from a sink for carbon to a source. In the tropics, corals are predicted to be wiped out by an increase in sea surface temperature of 2 degC. A cascade of tipping points could occur because the melting of Arctic sea ice amplifies heating by exposing dark ocean that absorbs more sunlight. That may increase the melting of Greenland ice and permafrost areas. Multiple risks can interact, with one change reinforcing another, and therefore, warming of just a degree or two is sufficient to result in dramatic cascading effects. Prof Martin Siegert, at Imperial College London, said: “The new work is valuable. They are being a little speculative, but maybe you need to be.” He also pointed out that the extremely rapid rate at which CO2 was being pumped into the atmosphere was unlikely to have ever occurred on Earth before. “It may mean that tipping points can occur in unexpected ways as there is no geological precedent for this rate of CO2 change.” The article reports that preliminary results from the latest climate models suggest global heating will be greater than expected, increasing the risk of tipping points. Prof Piers Forster, at the University of Leeds, disagreed on that point. However, he added: “I completely endorse their call for action. Although possibly low probability, the risks they identify are real.” Lenton said action would still have real benefits, by slowing the impacts and giving more time for people to adapt. He said: “This article is not meant to be a counsel of despair. If we want to avoid the worst of these bad climate tipping points, we need to activate some positive social and economic tipping points [such as renewable energy] towards what should ultimately be a happier, flourishing, sustainable future for the generations to come.”
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 On average, the global cloud amount is approximately 65-72% (counting cirrus). - International Satellite Cloud Climatology Project There’s more to cloud climatology than picking out what animal it most closely resembles. We can define a cloud as a visible region consisting of suspended particles of ice, liquid, or a combination of both. To form a cloud, we must increase the relative humidity such that it is greater than 100%. This can be done through radiative cooling or increasing the mixing ratio (i.e. the amount of the water vapor). There are 10 main cloud types formed from cirrus, stratus, and cumulus type clouds. Cloud climatology consists of defining clouds in terms of cloud amount, heights, cloud top temperature, optical depth, distribution, radiance, and lifecycle. Not all clouds precipitate either. (Precipitation is also intimately linked with global cloud climatology. Precipitation is typically associated with areas of rising air and low atmospheric pressure. The atmosphere itself is dramatically affected by latent heating associated with precipitation, an ultimate driver in overall precipitation patterns. Temperatures changes within the atmosphere also play a large role in the development and dissipation of clouds. Aerosols can act as cloud condensation nuclei, accelerating the rain process or inhibiting it).
The average cloud amount varies seasonally and with time of day, driven by solar radiation. Over land, cumulus tend to form in the early morning with dissipation in the early afternoon due to mixing. Cumulonimbus tend to take over in the late afternoon and evening hours in the continental summer, due to more organized forcing. The tropics have the highest total low cloud amount, while the total cloud amount is highest in the Northern Hemisphere winter associated with the midlatitude storm track. It is thought that high clouds could have an impact on warming and low clouds a net cooling effect, but these processes aren’t fully understood or use indirect observational evidence of low clouds. And yet, the Cloud-Radiative Forcing (CRF) experiment found that a climate system without clouds would have a net warming effect. Ground-based cloud observations have been largely subjective in determining cloud type and somewhat sparse over the ocean. Detecting clouds through satellite data often involves techniques, such as cloud masking to identify the presence of clouds through comparison of the brightness temperature of pixels (i.e. cold pixels will be clouds). Over polar regions, the cooler brightness temperatures of snow and ice-covered surfaces may possibly be mistaken for clouds. Although 3% uncertainty exists with cloud amount in climatology, ground-based stations and satellite comparison should be able to minimize potential sources of error in cloud climatology and help to better understand the role clouds play in a changing climate. To learn more about all things climate, please click here! ©2019 Meteorologist Sharon Sullivan |
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