Feedback, Water Vapour, Rising Sea Levels, and Melting Ice
Written by Neil McCabe
What is feedback?
Adding additional greenhouse gases to the atmosphere forces changes and responses in the climate system. A response known as feedback can either amplify the warming effect (positive feedback) or cause cooling, dampening the forcing effect (negative feedback). Plants and trees take CO2 out of the atmosphere and pump out O2, but these areas are massive water vapour creators. Forests act as “carbon sinks” and when we lose trees to deforestation, the atmosphere gets hotter creating more water vapour and a larger greenhouse effect.
The sun’s radiant heat is still getting through and the earth is getting hotter- therefore giving back out more heat as infrared quicker than the norm. The natural reaction is always the same- earth produces more water vapours. The greenhouse effect is now its own feedback, producing its own earth warming cycle.
The problem with ice caps melting is that the land beneath them becomes exposed and this brown newly exposed earth absorbs radiant heat from the sun, in turn this causes even more melting. This is a self-reinforcing cycle known as the Ice - Albedo feedback. People need to know about the plight of humans, due to ice feedbacks rather than the plight of polar bears and penguins. If ordinary people could relate flooding and the destruction of the home they live in to higher sea levels from earth's overheating due to manmade emissions – things would be different.
Water Vapour H2O
Due to increased energy consumption and pollution, greenhouse gases such as carbon dioxide in the atmosphere, increase earth’s temperature. Water vapour is an even more powerful greenhouse gas than carbon dioxide and is responsible for between 36% and 72% of the global greenhouse effect. But increasing water vapour occurs as a feedback to warming from the forcing of greenhouse gases such as CO2 into the atmosphere.
The warming allows the atmosphere to hold more water vapour and because of its abundance this causes more increases in temperature, amplifying the effects of the other greenhouse gases. Unlike CO2, water vapour spends a relatively short time spent in the atmosphere before it is converted to rain or sleet or snow and returns back to the land or sea.
The problem is that we are directly responsible for pumping an excessive amount of CO2 into the atmosphere from the burning of fossil fuels from the industrial revolution right up to today. The water vapour in the clouds does shield off solar radiation and that is a good thing but it also traps the heat from the earth’s surface (long wave radiation) causing a positive feedback loop. Again this has been happening for millions of years, but the greenhouse effect is being amplified by our interference in the gases entering the atmosphere.
Rising sea levels and Melting ice
In the early 1800’s the sea level noticeably started to rise. Prior to that earth had had a relatively steady period with little change in sea level. So from about 2000 years ago until the 19th century there was no real change. This was based on the period of melting ice exposing land, this feeds back to more ice melting faster. The ice has to go somewhere when it turns to water. Thermal expansion of the oceans is when warm water expands in the ocean and causes an increase in sea level. This expansion due to ocean warming is the dominant cause of sea level rise over the past 200 years.
An increasing contribution to sea level rise is ice sheet melt (Greenland and Antarctica) and most glaciers are retreating and getting smaller. Global warming is having serious dramatic effects on sea ice in the Arctic but sea ice melt has very little effect on sea level. There may have been some small growth in only a few locations but in general since 1980 it is well documented that there is a decrease in snow and ice on earth. Since 1966 the snow cover in spring in the northern hemisphere has reduced by 10%. That is a trend of 2% per decade. In the last 20 years sea level rise has averaged nearly 3mm per year, double the average rate of rise of the 20th Century.
I’ll be talking about ocean heat and storms, and ozone in the next blog.