Research Paper Global Warming Literature Review

Overall growth and growth in terms of disciplines and subfields

As a first step to provide an overview of the development of the entire research dealing with climate change, the time evolution of the publication productivity (output) of this research field (measured as number of papers published per year) has been analyzed. Fig 1 shows the annual number of papers published within the time period 1980–2014 and covered by the WoS database.

Fig 1

Time evolution of the overall number of climate change relevant papers (articles and reviews).

According to Fig 1, the total number of papers dealing with climate change shows a strong increase: Within the time period 1991 to 2010, the number of climate change papers published per year increased by a factor of ten, whereas in the same time period the overall number of papers covered by the WoS databases increased “only” by a factor of around two. The data row of Fig 1 exhibits a doubling of the climate change papers every 5–6 years. The exponential growth of climate change literature is possibly induced by the increasing influence of the IPCC Assessment Reports, which eventually made climate change research a hot topic. These reports revealed the strong need of further research for a better understanding of the earth’s climate system and for improved predictions of the future climate. Furthermore, the effects, impacts and risks of climate change became more and more concrete. The discussion of the question of human induced climate change towards a clear fact (at least for the majority of the scientific community, see Anderegg [2] stimulated research on future pathways for adaptation and mitigation.

The literature growth is roughly in accordance with the results of Grieneisen and Zhang [28], who report that the number of publications on climate change and global warming has doubled with a rate of approximately every 4 years. As mentioned above, Stanhill [5] found a doubling rate of 11 years for the time period 1951–1997. To put this into perspective, we compare our results with the growth rate of the overall science: According to Bornmann and Mutz [29] the total volume of publications covered by the WoS between 1980 and 2012 doubled approximately every 24 years. Hence, the growth rate of climate change related publications is extraordinarily high. The bend down around 2012 is presumably caused by still incomplete coverage of the recent publication years through WoS and is no sign of decline.

The results of the analysis of the climate change related papers with regard to their disciplines of origin are shown in Figs ​2 and ​3. The figures are based on the main OECD categories assigned. Compared to the WoS research areas and subject categories, the main OECD categories are broader grained and therefore better suitable for an overview.

Fig 2

Time evolution of the field-specific climate change related papers published since 1980, based on the main OECD categories assigned.

Fig 3

Field-specific relative increase of climate change related papers published since 1980, based on the main OECD categories (the number of papers published in 1980 equals 100%).

As expected, climate change research is dominated by the natural sciences. Further analyses of our data with regard to the specific research areas show that the earth sciences (meteorology and atmospheric sciences), the biological, the agricultural sciences, and the environmental sciences are predominant.

Fig 3 shows the relative increase of papers since 1980 assigned to the main OECD categories. The paper share is presented in percent increase based on the numbers from 1980 (thus, the number of papers published in the year 1980 in each case equals 100%).

According to Fig 4, climate change research has become an issue also for disciplines beyond the natural sciences (e.g. engineering, history, law, management, sociology etc.). The categories Engineering and Social Sciences show the strongest increase since around 2005. Since around 2009 the relative increase of the Natural Sciences and the Social Sciences is almost identical. Obviously, climate change is increasingly seen as a fact to be considered for the near future: The need to limit fuel combustion and to adapt to global warming apparently is a huge stimulation for various technological developments and research on the implications of climate change. For example, sociologists analyzed the public understanding and the discussion of climate change in science, politics, and the mass media [30, 31]. Furthermore, the Humanities have discovered climate change as a research topic. Historians for example reconstructed climate extremes in medieval history [32].

Fig 4

Time evolution of the papers of the major subfields within climate change research.

Climate change affects agriculture in a number of ways (changes in average temperatures, rainfall, climate extremes) and therefore has become an important field of investigation within climate change research. Climate change will likely affect food production and probably increases the risk of food shortage [3]. Although climate change is increasingly relevant also for medicine, the portion of output of this field of study is comparatively low.

The OECD categories are very broad and imply a classification of journals and not of the specific papers published therein. Also, many journals publish papers from different research fields and are assigned to more than one category which causes a certain amount of impreciseness. In order to differentiate our publication set with regard to the major subfields of climate change research we applied a different method: We parsed our paper set by combining with carefully selected further search terms which are based (1) on the title word analysis from step 1 of our literature search procedure and (2) on the major topics of climate change research as indicated by various summarizing publications (e.g. the IPCC Synthesis Report 2014, table of contents [1]).

As a first category of search terms, we selected the papers dealing with the main climate subsystems: the atmosphere, the oceanic water, the continental water, the ice sheets and glaciers, and the continental biomass. Additionally, we selected the literature specifically dealing with the various forms of atmospheric and oceanographic circulation or oscillation phenomena. All these search terms mark the kind of basic research in climatology, atmospheric- and geosciences, meteorology, and oceanography, which is undertaken to better understand the earth’s climate system. We completed this category by separately searching for the more theoretical publications dealing with climate modeling and the prediction of future climate. As a second category, we searched for papers dealing with the adaptation to climate change or its mitigation as well as papers focusing on effects, impacts, and risks of climate change. Such research takes climate change more or less as a matter of fact and discusses possible consequences and reactions. The corresponding terms were searched in titles and keywords only, because a search in abstracts might have resulted in too many false positives. For more detailed information of the search procedure, see the search terms in Table 1. Fig 4 shows the time evolution of the papers of the major subfields within climate change research.

According to Fig 4, continental biomass related research is the largest subfield within climate change research, closely followed by climate modeling, which demonstrates the importance of theoretical investigations (admittedly, these two subfields are rather broad). Next come research dealing with oceanic water, with impacts and effects of climate change, and with continental water (lakes, rivers, rainfall). Due to the radiative imbalance of the earth, less energy leaves the atmosphere than enters it. By far most of this extra energy has been absorbed by the oceans, which makes the oceans a major climate factor. The term “*sea*” was searched in addition to the terms “*ocean*” and “*marin*” to include papers dealing for example with changing sea surface temperatures or the rise of the sea levels into the answer set. Another major subfield is represented by the topic ice and snow (e.g. ice cores, ice sheets, glaciers, shelf ice). Ice cores are most important for the dating and reconstruction of the earth’s past climate as well as for the prediction of the future climate.

The next major subfield is related to the atmosphere as another important climate subsystem (beside the ice and water related subsystems). This subfield includes research on clouds, on wind and storms (i.e. the key topics of meteorology), but also on aerosols (see volcanic eruptions). In contrast to impacts and effects of climate change, which appear as a major field of interest, research dealing with the adaptation to and the mitigation of climate change as well as with the risks and the vulnerability of global warming are comparatively small. Both were next to insignificant until 2004, but their share of papers increased exponentially since 2005, showing the strongly increasing research activity in this field. Global warming also affects ocean currents and thereby periodical climate changes like ENSO (El Nino Southern Oscillation) and NAO (North Atlantic Oscillation). As a more specific research topic and a subset of the atmospheric and oceanic water subfields, this research represents the smallest topic within our publication set, thereby masking somewhat the importance of these research activities. In Table 2, the total number of papers as well as the bibliometric indicators PPTop 50%, PPTop 10%, and PPTop 1% of the papers belonging to the specific subfields are given.

Table 2

Major subfields of climate change research, ranked by publication output within the time period 1980–2012.

According to Table 2, all subfields together comprise more than 81% of the total climate change papers published within the time period 1980–2012 (note that we have restricted all citation impact analyses to 2012 as the most recent publication year). The PPTop 50% values of all subfields are above the proportion of the total climate change literature (PPTop 50% = 63.43%). Research on vulnerability (PPTop 1% = 3.51) and on adaptation (PPTop 1% = 3.47) can be seen as the subfields within climate change research publishing the largest proportion of very important papers.

Contributing countries

Climate change research is not only a highly multidisciplinary undertaking but also a research area with many countries being active and cooperating with each other. The number of papers of each country and their citation impact based on the PPTop 50% values are shown in Table 4, together with the percentage of excellent papers (i.e. PPTop 10% and PPTop 1%). The PPTopX% values in columns 2 are relative to the countries’ overall impact of all papers between 1980 and 2012. A value of 200 for example corresponds to twice the impact of the countries’ climate change papers compared to all the countries’ papers in the aforementioned time frame.

Table 4

Countries of authors ranked by publication output within the time period 1980–2012 (only countries with at least 1,000 papers are considered).

According to Table 4, research on climate change is quantitatively dominated by the USA, followed by the UK, Germany, and Canada. China appears on rank five, followed by France and Australia. PPTop 50% of these seven countries (with more than 10,000 papers in total) extends between 56.98% (China) and 74.59% (UK). PPTop 1% ranges from 1.6% (China) to 4.13% (UK). PPTop 10% ranges from 14.58% (China) to 26.13% (UK). Hence, the three citation-based indicators exhibit consistently that the UK has produced papers in climate change research with the largest reception compared to the other countries (with more than 10,000 papers). However, the other top countries rank nearby (with the exception of China, which nevertheless ranks above average). Switzerland, Denmark and also The Netherlands (with a publication output between around 3,000 and 6,000 papers) perform top with regard to all three bibliometric indicators–the impact of their contributions to climate change research is impressive. The citation impact of the climate change papers of all countries is above or far above the overall impact of the countries’ papers each.

Li et al. [6] presented a comparison of publication trends of the top seven most productive countries and found a quite similar ranking concerning the publication numbers (with only one exception: Australia appears on rank 5 compared to rank 7 in our publication output ranking).

Visualization of the time evolution of research topics

The maps presented in Fig 5 and Fig 6 show the title word clusters (clouds) of the climate change papers of the overall publication set (1980–2014) and of the papers from three specific publication time periods (1980–1990, 2003, and 2014). We have chosen these specific time periods in order to compare the papers of the most recent (complete) publication year (2014) with early publication years (1980–1990) and a publication year in between (2003). Due to the low number of papers per year before 1990 (caused by both the low publication output at that time and the lack of abstracts in WoS prior to 1991) we had to accumulate the early papers from a publication time period of about a decade (1980–1990). All title words of the same cluster appear as circles with the same color. The distance between the circles relates to the distance (or closeness) in terms of bibliographic coupling. The size of the circle is proportional to the number of papers found with these terms in the titles.

Fig 5

Title words from bibliographic coupling of climate change papers published within the overall time period 1980–2014.

Fig 6

Title words from bibliographic coupling of climate change papers published a) 1980–1990, b) 2003, and c) 2014. The minimum number of papers containing a specific title word is 10. The coloring of the clusters automatically performed by VOSviewer...

The most pronounced title words of the papers published within the overall time period 1980–2014 are climate change, effect, and impact (center). The red cluster (center right) includes papers related to energy and policy topics. Major title words are: climate change, adaptation, emission, framework, uncertainty, cost, technology, and policy (in the order of decreasing frequency). The blue cluster (center left) combines the papers around paleoclimate. Major title words are: record, year, variation, lake, sediment, and event. The green cluster (bottom left) contains theoretical publications. Major title words are: (climate) model, data, parametrization, and simulation (but also variability, which often appears in combination with climate modeling). The yellow cluster (center top) illustrates the importance of biological effects of global warming. Major title words are: effect, forest, soil, and plant. And finally, the magenta cluster (top) marks papers concerning acclimatization and survival of species.

If we analyze and compare the maps based on the three selected time spans (c.f. Fig 6) we find some remarkable changes: The title word map of the first decade (1980–1990, Fig 6A) shows the term climate as the most pronounced title word. The terms effect and influence appear secondarily. The term climatic change and the related terms appear third-rated (i.e. as small circles). The title word map constructed from papers published in the year 2003 (Fig 6B) for the first time accentuates the term change. The 2014 map (Fig 6C) is quite similar to the map of the overall publication set (Fig 5) with climate change and effect as the most pronounced terms. The reader might miss the term impact, but it is hidden behind climate change.

The changing title words based on the maps of the three specific publication times exhibit that the term climate change comes forward with time. Obviously, the authors increasingly use a term which implies global warming (and therewith anthropogenic causes) as a matter of fact. Furthermore, the term impact arises and points to research dealing with the various effects and risks of climate change–see also the IPCC Synthesis Report 2014, Summary for Policymakers [3]. The term model and related terms (e.g. simulation) appear independently of time. This indicates the high relevance of climate modeling since the beginning of the time period analyzed here.

Today, the most comprehensive analysis of peer-reviewed climate research to date was published in the journal Environmental Research Letters. Our analysis found that among papers expressing a position on human-caused global warming, over 97% endorsed the consensus position that humans are causing global warming. Overwhelming agreement among scientists had already formed in the early 1990s. And the consensus is getting stronger.

In a previous Conversation article, I argued that climate denial is essentially consensus denial. For over two decades, attacking the scientific consensus has been a central part of the movement to prevent meaningful climate action.

As early as 1991, Western Fuels Association spent $510,000 on a campaign to “reposition global warming as theory (not fact)”. Their strategy was to construct the impression of active scientific debate using dissenting scientists as spokesmen. This approach was concisely articulated in a memo to Republicans by political strategist Frank Luntz, leaked in 2002:

Voters believe that there is no consensus about global warming in the scientific community. Should the public come to believe that the scientific issues are settled, their views about global warming will change accordingly. Therefore, you need to continue to make the lack of scientific certainty a primary issue in the debate.

Using Skeptical Science’s taxonomy of climate myths, a recent analysis tracked climate misinformation published in opinion editorials from 2007 to 2010 by syndicated conservative columnists. The most popular myth was “there is no consensus”. More recently, a variation of the “no consensus” myth has emerged – the notion that the consensus is “on the verge of collapse”.

Our analysis examined the status of the scientific consensus over 21 years of published climate research, from 1991 to 2011. We searched for any papers matching the search “global warming” or “global climate change” in the Web of Science, a database of scientific peer-reviewed research. We rated the level of endorsement of human-caused global warming in each abstract, a short summary at the start of each paper.

In 2007, Naomi Oreskes predicted that as a consensus forms, fewer papers should explicitly endorse the consensus position. For example, you don’t expect to see geography research papers endorsing the fact that the earth is round. Our analysis confirmed this prediction, finding most abstracts didn’t state a position on whether humans were causing global warming.

However, we did identify over 4,000 abstracts that did state a position on human-caused global warming. Among those 4,000 abstracts, 97.1% endorsed the consensus. There was overwhelming agreement on human-caused global warming in every year since 1991.

To independently check our results, we also invited the thousands of scientists who authored the climate papers to rate the level of endorsement of their own papers. We received 1,200 responses with over 2,000 papers receiving a “self-rating”. Interestingly, most of the abstracts that we rated as “No Position” turned out to endorse the consensus in the full paper, according to the papers’ authors. Among all the papers that were self-rated as expressing a position on human-caused global warming, 97.2% endorsed the consensus.

Our results are strikingly consistent with other measurements of consensus. The seminal work on consensus was conducted by Naomi Oreskes who in 2004 analysed 928 climate papers. She found zero papers rejecting the consensus. We analysed the same papers as Oreskes and similarly found zero rejections in the papers matching her search parameters.

Two more recent studies have sought to measure the level of consensus in the scientific community. A survey of Earth scientists found that among actively publishing climate scientists, 97% agreed that humans were significantly changing global temperature. A compilation of scientists making public statements on climate change found that for the scientists who had published peer-reviewed climate research, there was 97% agreement.

While a number of studies have independently established overwhelming agreement among climate scientists, two decades of sustained attack on the consensus has been effective. There is a gaping chasm between the public perception and the actual 97% consensus. When a US representative sample was asked how many climate scientists agree that humans are causing global warming, the average answer was around 50%.

Why is climate denial synonymous with consensus denial? Social scientists are just starting to figure out what climate deniers have understood for decades. A 2011 study found that when people correctly understand that climate scientists agree, they are more likely to support policy to mitigate climate change. This is why a political operative hired by fossil fuel interests to undermine climate policy focused on attacking the consensus, arguing “If we win the science argument, it’s game, set, and match.”

This underscores the importance of correcting the mis-perception that scientists are still debating whether humans are causing global warming. An important step towards stronger public support for meaningful climate action is closing the consensus gap.

The results of the paper Quantifying the Consensus on Anthropogenic Global Warming in the Scientific Literature are summarised in a simple, user-friendly manner at theconsensusproject.com.

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