Now, for sake of reading, I've devided this post into three sections. Section 1 deals with proving that the Earth is warming. If your alread believe the Earth is warming, then go ahead and skip to section 2. And Section 3 is the references.
So, without further ado, let's get this **** started:
SECTION I: Arguements against Global warming and debunking them.
What was the scientific consensus in the 1970s regarding future climate? The most cited example of 1970s cooling predictions is a 1975 Newsweek article "The Cooling World" that suggested cooling "may portend a drastic decline for food production."
An ice age was predicted in the 1970's.
"Meteorologists disagree about the cause and extent of the cooling trend… But they are almost unanimous in the view that the trend will reduce agricultural productivity for the rest of the century."A 1974 Time magazine article Another Ice Age? painted a similarly bleak picture:
"When meteorologists take an average of temperatures around the globe, they find that the atmosphere has been growing gradually cooler for the past three decades. The trend shows no indication of reversing. Climatological Cassandras are becoming increasingly apprehensive, for the weather aberrations they are studying may be the harbinger of another ice age."Peer-Reviewed Literature
However, these are media articles, not scientific studies. A survey of peer reviewed scientific papers from 1965 to 1979 show that few papers predicted global cooling (7 in total). Significantly more papers (42 in total) predicted global warming (Peterson 2008). The large majority of climate research in the 1970s predicted the Earth would warm as a consequence of CO2. Rather than 1970s scientists predicting cooling, the opposite is the case.
Figure 1: Number of papers classified as predicting global cooling (blue) or warming (red). In no year were there more cooling papers than warming papers (Peterson 2008).Scientific Consensus
In the 1970s, the most comprehensive study on climate change (and the closest thing to a scientific consensus at the time) was the 1975 US National Academy of Sciences/National Research Council Report. Their basic conclusion was "…we do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate…"This is in strong contrast with the current position of the US National Academy of Sciences: "...there is now strong evidence that significant global warming is occurring... It is likely that most of the warming in recent decades can be attributed to human activities... The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action." This is in a joint staetment with the Academies of Science from Brazil, France, Canada, China, Germany, India, Italy, Japan, Russia and the United Kingdom.In contrast to the 1970s, there are now a number of scientific bodies that have released statements affirming man-made global warming. Here's some other sites:
Reasoning Behind Cooling Predictions
Quite often, the justification for the few global cooling predictions in the 1970s is overlooked. Probably the most famous such prediction was Rasool and Schneider (1971):
"An increase by only a factor of 4 in global aerosol background concentration may be sufficient to reduce the surface temperature by as much as 3.5°K."Yes, their global cooling projection was based on a quadrupling of atmospheric aerosol concentration. This wasn't an entirely unrealistic scenario - after all, sulfur dioxide (SO2) emissions were accelerating quite rapidly up until the early 1970s (Figure 2). These emissions caused various environmental problems, and as a result, a number of countries, including the USA, enacted SO2 limits through Clean Air Acts. As a result, not only did atmospheric aerosol concentrations not quadruple, they declined starting in the late 1970s:
Figure 2: Global sulfur dioxide emissions by source (PNNL)Similarly, if we now limit CO2 emissions, we can also eventually get global warming under control.
So global cooling predictions in the 70s amounted to media and a handful of peer reviewed studies. The small number of papers predicting cooling were outweighed by a much greater number of papers predicting global warming due to the warming effect of rising CO2. Today, an avalanche of peer reviewed studies and overwhelming scientific consensus endorse man-made global warming. To compare cooling predictions in the 70s to the current situation is both inappropriate and misleading. Additionally, we reduced the SO2 emissions which were causing global cooling. The question remains whether we will reduce the CO2 emissions causing global warming.
If there's one thing that all sides of the climate debate can agree on, it's that climate has changed naturally in the past. Long before industrial times, the planet underwent many warming and cooling periods. This has led some to conclude that if global temperatures changed naturally in the past, long before SUVs and plasma TVs, nature must be the cause of current global warming. This conclusion is the opposite of peer-reviewed science has found.
The Earth has warmed before!
Our climate is governed by the following principle: when you add more heat to our climate, global temperatures rise. Conversely, when the climate loses heat, temperatures fall. Say the planet is in positive energy imbalance. More energy is coming in than radiating back out to space. This is known as radiative forcing, the change in net energy flow at the top of the atmosphere. When the Earth experiences positive radiative forcing, our climate accumulates heat and global temperature rises (not monotonically, of course, internal viability. will add noise to the signal).
How much does temperature change for a given radiative forcing? This is determined by the planet's climate sensitivity. The more sensitive our climate, the greater the change in temperature. The most common way of describing climate sensitivity is the change in global temperature if atmospheric CO2 is doubled. What does this mean? The amount of energy absorbed by CO2 can be calculated using line-by-line radiative transfer codes. These results have been expirementall confirmed by satellite and surface mesurments. The radiative forcing from a doubling of CO2 is 3.7 Watts per square metre (W/m2) (IPCC AR4 Section 2.3.1).
So when we talk about climate sensitivity to doubled CO2, we're talking about the change in global temperatures from a radiative forcing of 3.7 Wm-2. This forcing doesn't necessarily have to come from CO2. It can come from any factor that causes an energy imbalance.
How much does it warm if CO2 is doubled? If we lived in a climate with no feedbacks, global temperatures would rise 1.2°C (Lorius 1990). However, our climate has feedbacks, both positive and negative. The strongest positive feedback is water vapour. As temperature rises, so too does the amount of water vapour in the atmosphere. However, water vapour is a greenhouse gas which causes more warming which leads to more water vapour and so on. There are also negative feedbacks - more water vapour causes more clouds which can have both a cooling and warming effect.
What is the net feedback? Climate sensitivity can be calculated from empirical observations. One needs to find a period where we have temperature records and measurements of the various forcings that drove the climate change. Once you have the change in temperature and radiative forcing, climate sensitivity can be calculated. Figure 1 shows a summary of the peer-reviewed studies that have determined climate sensitivity from past periods (Knutti & Hegerl 2008).
Figure 1: Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thick coloured bars indicate likely value (more than 66% probability). The thin coloured bars indicate most likely values (more than 90% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) and most likely value (3°C) are indicated by the vertical grey bar and black line, respectively.
There have been many estimates of climate sensitivity based on the instrumental record (the past 150 years). Several studies used the observed surface and ocean warming over the twentieth century and an estimate of the radiative forcing. A variety of methods have been employed - simple or intermediate-complexity models, statistical models or energy balance calculations. Satellite data for the radiation budget have also been analyzed to infer climate sensitivity.
Some recent analyses used the well-observed forcing and response to major volcanic eruptions during the twentieth century. A few studies examined palaeoclimate reconstructions from the past millennium or the period around 12,000 years ago when the planet came out of a global ice age (Last Glacial Maximum).
What can we conclude from this? We have a number of independent studies covering a range of periods, studying different aspects of climate and employing various methods of analysis. They all yield a broadly consistent range of climate sensitivity with a most likely value of 3°C for a doubling of CO2.
The combined evidence indicates that the net feedback to radiative forcing is significantly positive. There is no credible line of evidence that yields very high or very low climate sensitivity as a best estimate.
CO2 has caused an accumulation of heat in our climate. The radiative forcing from CO2 is known with high understanding and confirmed with empirical observation. The climate response to this heat build-up is determined by climate sensitivity.
Ironically, when skeptics cite past climate change, they're in fact invoking evidence for strong climate sensitivity and net positive feedback. Higher climate sensitivity means a larger climate response to CO2 forcing. Past climate change actually provides evidence that humans can affect climate now.
For sake of viewing, will continue in later posts.