There is an overwhelming scientific consensus that climate change is not only happening but also caused by humans. I think most people agree on this, although more so on the former. Perhaps, people have a basic understanding of how global warming works, and/or have noticed increasing extreme weather events lately such as record highs, or Hurricane Harvey¹. But, do most people understand the underlying scientific evidence pointing to human-caused global warming? Admittedly, I don’t, despite endorsing the consensus.

I’m starting my PhD next semester which focuses on the intersection of public health, food systems, and climate change. To be able to build on the quality work of others, it’s time that I fully understand the climate change issue by exploring the evidence myself and forming my own opinion. What better way to study climate change than by reading the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), a peer-reviewed synthesis of peer-reviewed scientific research (IPCC, 2015).

Why the IPCC report

The IPCC, established in 1988, is the leading international body for assessing the science related to climate change. Its purpose is to provide policymakers with regular assessments of climate change – the impacts and risks, along with ways to address climate change. The report was written by hundreds of leading scientific, technical, and social-economic experts who volunteered their time and expertise. Many other experts contributed their expertise to specific sections, while other experts acted as reviewers to ensure that the report reflects the full range of views in the scientific community. In short, the report appears to be credible, transparent, and objective.


I only read the Summary for Policymakers. My first impressions – an impressive accomplishment of synthesizing thousands of pages in the original report to merely 30 pages. I like the balance between content, white space, and figures. Adding main messages in headings were also a nice touch.

Although these messages have been said for decades, there were many takeaways for me. To highlight a few:

Global warming evidence and impacts

  • Greenhouse gas emissions have increased since pre-industrial era
  • Economic growth and population growth are most important drivers of emissions
  • Leads to an average warming of 0.85°C since the 1880s
  • Each of the last three decades has been successively warmer
  • Changes observed in many extreme weather events since the 1950s
  • Without any mitigation, the temperature can rise to 4°C by 2100

Evidence of human contribution

  • Atmospheric CO2 contains information about its source, allowing scientists to tease apart natural and human contributions
  • Nearly all CO2 emissions come from human contributions
  • Climate models with natural factors and human factors show a geographic pattern of temperature change similar to what has occurred recently
  • Near linear relationship between cumulative CO2 emissions and global temperature change


I have several concerns. First, it was written by science people, intended for non-science people. As stated in preface vii, “To facilitate access to the findings of the Synthesis Report for a wide readership and to enhance their usability for stakeholders, each section of the Summary for Policy Makers carries highlighted headline statements… the headline statements provide an overarching summary in simple and completely non-technical language for easy assimilation by readers from different walks of life”.

Hmm, not quite. Within the first couple pages of the Summary for Policymakers, heading 1.1 reads “Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia…”. The headline still seems too technical and not appropriate for “different walks of life”. Indeed, a study published in Nature Climate Change found that the report clearly stand out in terms of low readability (Barkemeyer et al. 2016). The report could benefit from having non-scientific experts revise statements, along with the body of content, for more accessibility.

Secondly, the criterion for evaluation is a bit unclear. It reads “Each finding is grounded in an evaluation of underlying evidence and agreement… In many cases, a synthesis of evidence and agreement supports an assignment of confidence.” Does this mean high evidence and high agreement = high confidence? Also, as I read through, there were many instances where a finding was not backed up by a confidence value, leading me to question whether its absence was due to low confidence which was not explicitly reported.

Thirdly, adaptation and mitigation –  there are a lot of talks about their benefits, but I find their action statements to be a bit weak. I understand that the report aims to be politically neutral, but it would be more actionable to explicitly state which countries need to take action, and by how much.

“Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge with existing practices increases the effectiveness of adaptation” (Page 19). Kudos to the team for recognizing the value of traditional knowledge in the body of text. However, to better convince policymakers, I suggest recognizing this value in the heading, which frankly, is the only part that most people read.

Final thoughts

Overall, the Summary for Policy Makers provides convincing evidence of human-caused global warming. You may wonder, why bother reviewing the evidence anyway, especially from a credible source? It takes too much time and mental energy, whereas a more attractive option is to read a 1-2 page summary of some news article. My response? It depends on your purpose. Personally, I want to think for myself, and after doing this research, I feel more comfortable agreeing with the consensus. Going further, I feel more motivated to take steps to address climate change through my PhD research. Indeed, we are all more likely to take action if we ‘feel’ for the message.

How will you respond?

¹Note that hurricanes are complex and naturally occurring. While climate change may have played a role in worsening the impact of Hurricane Harvey, the extent of contribution is hard to tease out.

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