Humans do impact climate change
Thomas Crawford (“Humans are not causing global warming,” Jan. 9) mistakenly contends that global warming in the industrial age is part of the manufacturing, normal cycle of climate change that the Earth has always experienced. Physical scientists know better, and the evidence for humans impacting global warming is clear and well established.
First, no scientist denies that carbon dioxide is a greenhouse gas or that greenhouse gases in the lower atmosphere trap heat. We have quantified the increase of carbon dioxide in the atmosphere with great accuracy for over a century and with adequate accuracy since before Homo sapiens spread across the globe. There is a strong correlation between global temperature and the amount of atmospheric carbon dioxide that holds over hundreds of thousands of years.
Without exception, top international panels such as the International Panel on Climate Change and national organizations like the National Academy of Sciences have concluded that recent climate change is due to human activities. The pattern of observed warming, with greater warming over land than over the ocean supports this conclusion. Computer model simulations of the temperature increase over time achieve a “best match” when human causes (anthropogenic forcings) are added to natural effects.
Unlike previous periods in Earth’s climate history, the level of carbon dioxide in the Earth’s atmosphere is at a historic high, unmatched over the last 100,000 years and the rate of temperature rise is also at an historic high. The information from the distant past comes from several methods, including ice core samples, tree ring studies, marine sediments and soil samples. The hottest decade in recorded history is the 2010-2019 decade which surpassed the previous high of the 2000-2009 decade. Guess what the record was before that? Right, the 1990s.
Furthermore, we understand the reasons for this extreme global warming (and climate change). Human activities including burning fossil fuels, emissions from our smokestacks and tailpipes, agriculture, and destruction of environmental carbon sinks (e.g., plant, animal, and tree destruction from fires in the Amazon, California and Australia) that would normally remove carbon dioxide from the atmosphere have all contributed to the increased temperature and increased presence of carbon dioxide in the atmosphere.
Denying the consensus of climate scientists and top experts is an example of willful ignorance. Ignoring this existential problem imperils future generations. We must act now.
Global carbon cycle can’t be disregarded
Thomas Crawford is again misquoting climate science, with his only true statements being how plants utilize CO2 and the percentages of the gases in our atmosphere.
First, the Columbia University survey that he misquotes is a paper, “Examining the Scientific Consensus on Climate Change,” Doran and Zimmerman, 2009. The survey had 3,146 yes or no responses to the question, “Do you think human activity is a significant contributing factor in changing mean global temperatures?” while also recording expertise in climate science of respondents. In it, 97.5% of published expert climate scientists answered yes, where only 58% of the general public answered yes.
The authors conclude: “It seems that the debate on the authenticity of global warming and the role played by human activity is largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes. The challenge, rather, appears to be how to effectively communicate this fact to policy makers and to a public that continues to mistakenly perceive debate among scientists.”
Second, he states that the Earth was warmer during several periods in the three previous millennia; however, a 2013 paper in Nature titled, “Continental-scale temperature variability during the past two millennia,” concludes that the “area-weighted average of the best estimate of past temperature from all seven regions indicates that 1971-2000 was warmer than any other time in nearly 1,400 years ... the global warming that has occurred since the end of the nineteenth century reversed a persistent long-term global cooling trend. The increase in average temperature between the nineteenth and twentieth centuries exceeded the temperature difference between all other consecutive centuries in each region, except Antarctica and South America.”
Third, he states that only 2% to 5% of CO2 comes from humans, but he disregards the global carbon cycle. He is only looking at emissions from the oceans and land. Before the onset of widespread fossil fuel use, the CO2 content in the air remained steady for thousands of years, with CO2 being emitted from and absorbed by the land and ocean, i.e. CO2 expelled by animals and absorbed by plants.
This natural process still exists today; however, humans are pumping out CO2 at an ever-increasing rate, disrupting that natural cycle. Between 1751 and 1987 burning fossil fuels put about 737 billion tons of CO2 into the atmosphere. Between just 1987 and 2014, it was 743 billion tons. Total CO2 from industrialized humans in the past 263 years: 1,480 billion tons. This increase in CO2 is what is causing the problem, not the carbon cycle. “Global Warming Fact: More than Half of All Industrial CO2 Pollution Has Been Emitted Since 1988,” Frumhoff, 2014.
People should trust scientists, scientific process
Thomas Crawford made a variety of statements and claimed they are scientific facts that prove humans are not contributing to climate change. I sympathize with his position and others who think similarly because there are a lot of mixed messages on this subject. Many of our government leaders refute or downplay humans’ role in climate change, and many people have personal beliefs that release humans from blame. However, Mr. Crawford’s comments are misleading and deserve a rebuttal.
If there was no word limit I would spend time refuting the letter’s comments about CO2 and state that scientists “did the math” and small amounts of atmospheric CO2 indeed have a significant effect. However, the most influential comment in the letter describes a survey seemingly showing only a tiny percentage of scientists believe in “dangerous” global warming.
The 2009 survey Crawford cited is actually from the University of Illinois in Chicago (authored by Doran and Zimmerman), not Columbia, and is an often misquoted by climate skeptics. Crawford, like others, claimed that of the 3,148 respondents, “77 said the world was warming dangerously.” The 77 respondents came from a narrow subgroup of 79 scientists (out of the 3,148) who “listed climate science as their area of expertise, and who also have published more than 50% of their recent peer-reviewed papers on the subject of climate change.” When looking at all 3,148 responses, 82% of scientists “agreed that humans significantly influence the global temperature.”
I have worked alongside climate scientists and astrophysicists as a computer scientist for 25 years helping to gather, analyze and publish scientific information. People should understand that refereed, scientific papers are THE body of scientific knowledge. They are devoid of opinions and are only accepted by the scientific community if their results can be verified by other scientists.
The main point I want to make in this letter is that science is naturally self-correcting. Scientists’ reputations are primarily based on the popularity (measured by the number of citations) of their published papers. If a scientist could show there’s verifiable evidence that humans are not contributing to global warming, they would immediately rise in prominence and additional, similar papers would follow. This hasn’t happened. I urge people to trust the scientific process that has overwhelmingly agreed that humans are causing warming of the Earth.