Long-term Warming in Climate Context

Human-Caused Climate Disruption in the Context of Natural Variability


When it comes to climate change science, we often fail to see the forest for the trees. With a complex array of variables like cloud patterns, ocean circulation, solar cycles, volcanic eruptions, and more, the climate system requires a tremendous degree of perspective across time and space and is therefore very difficult to observe. Without this perspective, we tend to focus on specific weather events or short-term trends that do not necessarily represent the overall climate pattern. It has fallen upon climate scientists to develop the technical sophistication and to employ the research methods necessary to understand climate variables, how they change across time and space, and the degree of human-caused climate disruption.

Table of Contents

The consensus of climate scientists around the world is communicated in the IPCC’s Fifth Assessment Report (AR5). Working Group 1 covers the physical science basis of climate change and cites more than 9,200 peer-reviewed journal articles. It states with 95% certainty that more than half of the observed increase in global average surface temperatures since 1950 is due to human-caused increases in greenhouse gases. It also states that the decade from 2001-2010 was the hottest on record. Scientists have noted that this record was set despite a convergence of natural factors exerting cooling influences, further demonstrating the strength of the human influence on our warming climate. Scientists make these assessments on the basis of extensive research and employ both observational data and model predictions across varying timescales.

The sections below discuss several topics that underscore long-term climate trends, including: the reality that recent decades are progressively hotter with the most recent decade the hottest on record; the scientific community’s certainty that warming trends are predominately human-caused; and the importance of modeling to understanding climate. Each section includes relevant quotes from the AR5 and scientific experts. 

The Hottest Decade: Overcoming Natural Cooling Factors

It’s the hottest decade ever, even with a double La Niña and a deep solar minimum. These cooling factors won’t last.

Looking at the complete global picture, one that includes ocean heat content as well as surface and atmospheric warming trends, what we find is striking. Not only is global warming still rising, but recent temperatures are also unprecedented at a time when natural indicators alone promote cooling.

Thirteen out of fifteen of the hottest years on record occurred since 2000, and the two exceptions (1997 and 1998) were strong El Niño years. The decade from 2001-2010 was the warmest since thermometer records began in 1850 and probably going back further. Temperature reconstructions dating back 11,000 years show that modern temperatures are among the warmest human civilization has ever experienced, and that the modern rate of warming is exceptional, likely unprecedented in that time period.

Despite this overwhelming evidence of the unusual nature of modern temperatures, many have focused on a reduction in the rate of surface warming over the past decade as a sign that global warming might not be so bad. However, global temperature datasets are not able to capture warming in difficult to measure places like the Arctic, which has been warming faster than the rest of the world, and they do not capture warming beneath the surface, such as in the deep ocean. For these reasons, a recent study reveals that one of the major global temperature records, HadCRUT4, used by organizations like the United Kingdom National Weather Service may be underestimating global surface temperature. The study authors reconstructed the data using a hybrid method that incorporates modern satellite information and found that since 1997 the globe has been warming 2.5 times faster than the HadCRUT4 temperature records show.

Science commentators have also highlighted an important bigger picture: natural factors that should have been cooling our world for the past decade or more have instead only slightly slowed the rate of warming. This means that humans, rather than natural factors, are behind the wheel steering the course of the global climate.

The natural factors that should have been cooling the planet, if not for human influence, include:

Rather than letting wishful thinking convince us that surface warming has “paused,” we should be asking: how much warming will we see, as we continue to pump more and more carbon into the atmosphere and the planet’s natural cooling effects cease?

In the approximately 150 years since we began measuring surface temperatures with thermometers, temperatures haven’t risen upwards smoothly – they’ve moved in a staircase pattern, with “steps” lasting a decade or more. History tells us that once current natural cycles revert, surface warming will jump upward (as it did in 1998). This shows us the true and sobering fingerprint of human influence.

Relevant Quotes from Scientists:

Dr. Michael Mann, Professor of Meteorology and Director of the Earth System Science Center at Pennsylvania State University:

  • “Natural factors such as volcanoes and reduced solar output should have led to a cooling trend in recent decades. If you talk to most scientists who work on this, they would tell you that more than 100% of warming is due to human activity.” (CN Interview)

Dr. Andrew Pitman, Director of the Australian Research Council’s Centre of Excellence for Climate System Science:

  • “The slowing in the rate of warming over the last 15 years is not in the least surprising. We have seen a combination of the solar minimum, anthropogenic aerosol emissions and back-to-back La Niñas. What is surprising – and what is deeply concerning to me and almost entirely missed in the media commentary – is that we have not cooled dramatically over the last 15 years.” (Blog Post)

Dr. Colin Summerhayes, Scott Polar Research Institute and Reviewer of the IPCC WGI chapter on Paleoclimate:

  • “We are warming when we should be cold … The new [IPCC] report shows yet again that the global warming we are seeing today cuts right across what we would expect from our knowledge of climate change over the past 11,000 years.” (Science Media Centre)

What the AR5 says:

Human attribution

  • “It is extremely likely [all emphases original] that more than half of the observed increase in global average surface temperature from 1951 to 2012 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period.” (SPM-12)

Short-term surface temperature variability

  • “In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and interannual variability (see Figure SPM. 1). Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming in the past 15 years (1998-2012; 0.05 [-0.05 to +0.15] ºC per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951-2012; 0.12 [0.08 to 0.14] ºC per decade).” (SPM-3)

Useful visuals:

This graph ranks the annual global mean surface temperature in the HadCRUT4 dataset for each year since 1850. Bars are color-coded by time period, and the length of each bar indicates the observation uncertainty. Source: HadCRUT4, Met Office.

Global annual average temperature anomalies (relative to the 1961-1990 average) for 1950-2013 based on an average of the three data sets from NASA, NOAA and the UK Met Office. Coloring indicates whether a year was classified as an El Niño year (red), an ENSO neutral year (grey) or a La Niña year (blue). Source: Climate Central, WMO.

Warming Continues: Understanding Surface vs. Global Warming

Not only have we seen the hottest decade on record, but the overall warming trend is also steadily continuing.

This point often gets lost in the focus solely on surface warming trends, an approach that misses many important indicators showing that global warming hasn’t stopped or even slowed. Top points to note include:

  • Oceans are continuing to absorb heat, especially the deep oceans.
  • Sea ice melt, ice sheet melt, glacier melt and sea level rise are on the high end of IPCC predictions, and they are accelerating.
  • The only meaningful way to measure warming is globally and over the long term.

The “warming has stopped” voices ignore the vast number of indicators other than surface temperature that are equally important to the climate system. Studies estimate that over 90% of the heat reaching the Earth is absorbed into the oceans, and that ocean warming has continued unabated, or even increased. In fact, the warmer oceans are melting Antarctic ice shelves from below.

Other indicators include the Greenland ice sheet, which is losing mass at an accelerating rate, and Arctic sea ice, which has reached shocking summer extents even lower than IPCC predictions. In 2007 sea ice retreated so much that the Northwest Passage opened for the first time in human record. Global sea levels have also risen faster than IPCC predictions.

The continuation of sea level rise, ocean warming, and ice melt around the world are signs that warming hasn’t stopped; it’s been redistributed. When faced with a more complete global picture, it’s clear just how misleading it is to focus solely on short-term surface warming trends.

Relevant Quotes from Scientists:

Dr. Noah Diffenbaugh, Associate Professor in the School of Earth Sciences and Senior Fellow at the Woods Institute for the Environment at Stanford University:

  • “Has warming stopped? No. Does the warming trend over the past few years mean that climate models were wrong? No.” (Testimony for the Safe Climate Caucus)

Dr. Richard Allan, Professor of Climate Science at the University of Reading:

  • “Although global surface warming has slowed over the last 15 years, observations show that heat has continued to accumulate within the oceans since 2000 – at a rate equivalent to over 250 billion 1kiloWatt electric heaters spread across the planet – consistent with rising atmospheric concentrations of greenhouse gases.” (Science Media Center)

Dr. Michael Mann, Professor of Meteorology and Director of the Earth System Science Center at Pennsylvania State University.

  • “By many measures, whether you’re looking at Arctic sea ice, heat content of the ocean, global sea level rise, etc., climate change is proceeding either at or ahead of schedule relative to the model predictions.” (CN interview)

What the AR5 says:

Arctic sea ice melt

  • “The annual mean Arctic sea ice extent decreased over the period 1979-2012 with a rate that was very likely in the range 3.5 to 4.1% per decade (range of 0.45 to 0.51 million km2 per decade), and very likely in the range 9.4 to 13.6% per decade (range of 0.73 to 1.07 million km2 per decade) for the summer sea ice minimum (perennial sea ice).” (SPM-6)

Greenland ice sheet melt

  • “The average rate of ice loss from the Greenland ice sheet has very likely substantially increased from 34 [-6 to 74] Gt yr-1 over the period 1992-2001 to 215 [157 to 274] Gt yr-1 over the period 2002-2011.” (SPM-5)

Antarctic ice sheet loss

  • “The average rate of ice loss from the Antarctic ice sheet has likely increased from 30 [-37 to 97] Gt yr-1 over the period 1992-2001 to 147 [72 to 221] Gt yr-1 over the period 2002-2011.” (SPM-5)

Glacial melt

  • “The average rate of ice loss from glaciers around the world, excluding glaciers on the periphery of ice sheets, was very likely 226 [91 to 361] Gt yr-1 over the period 1971-2009, and very likely 275 [140 to 410] Gt yr-1 over the period 1993-2009.” (SPM-5)

Sea level rise

  • “It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm yr-1 between 1901 and 2010, 2.0 [1.7 to 2.3] mm yr-1 between 1971 and 2010 and 3.2 [2.8 to 3.6] mm yr-1 between 1993 and 2010.” (SPM-6)

Useful Visuals:

Short vs. Long Trends

This figure shows how short-term trends in global temperature (here, the blue lines that show temperature trends at five-year intervals from 1970 to 2010) can range from decreases to sharp increases. The evidence of climate change is based on long-term trends over 20-30 years or more (red line). Source: measurement data from NASA-GISS, National Climate Assessment Draft.

Ocean Heat Graph

This figure shows how the ocean has absorbed the majority of increases in total earth heat content since 1950. Source: Murphy 2009, ocean data taken from Domingues et al 2008.

Climate Models: Integrating Across Time and Space

Climate models work best when integrating many factors across longer time periods and wider spaces, including the Earth’s surface, oceans and atmosphere.

In the absence of a “control Earth” upon which to perform experiments, scientists often rely on models as one of many tools to understand the Earth’s climate system. Thanks to modern computing power and technology, scientists can develop models to analyze climate conditions across time and space. Using models, scientists can better understand the dynamics of factors like cloud patterns, ocean circulation, solar cycles, and volcanic eruptions as well as the impact of human-caused increases in greenhouse gases. Models can show us, for example, what a climate without human influence might look like, or what extremes might exist in a much warmer future climate.

Recently, however, some have wondered whether the current generation of climate models conform accurately enough to observed trends. Papers such as Fyfe et al. have amplified these concerns by showing that over the past 20 years, models predicted warming at a greater rate than observed surface temperatures show. Detractors argue, “If the models aren’t accurate over 10 years, how are they any good at all? Why should we pay any attention to their projections over 100 years?”

Since the publication of Fyfe et al, Cowtan and Way have shown that model predictions may not be as far off from observational data as originally thought. Despite the remaining discrepancy, there are three main reasons why models are useful indicators for measuring climate change:

  • When we focus exclusively on a single measurement of modeled output, we’re not getting the whole story. In this case the focus on overestimated surface warming misses a clear counterpart: underestimated ocean warming and volcanic output. The total “global” warming is continuing.
  • Models function better over long timescales than shorter ones due to the short-term effects of natural variation. They can often predict the cumulative effect of disruptive events but not the timing of individual events.
  • Models give us insight we can’t get in other ways. Because we don’t have a “control Earth,” some experiments can only be run using models. They are one tool in our toolbox for understanding the climate — not the foundation of all climate science conclusions. Scientists compare the output of models with observations and with what we already know about the climate.

By focusing solely on surface warming as the proving ground for climate models, we miss the bigger picture that models reliably predict, and we fail to see the continuing global warming trend. If we want to truly compare the models’ projections to “global” warming, we need to take the oceans into account. Scientists are increasingly doing this, and seeking to improve the models’ accounting of ocean heat and long-term ocean cycles. Measurement of the ocean, especially the deep ocean, is a rapidly advancing frontier of climate science.

New research such as Guemas et al., Balsameda et al. and Kosaka et al. show that long-term cycles may transfer warming in and out of the ocean’s depths, and reductions in surface warming may be the counterpart of increases in ocean warming. This means that along with overestimating recent surface warming, models may have underestimated recent ocean warming. Evidence bears this out. Arctic sea ice extent and thickness have declined faster than models predicted, and sea level has also risen faster than models predicted. Both trends are the symptoms of a warming ocean.

Volcanic output may have been underestimated as well. New studies point to the fact that even small eruptions can have an impact on the climate, not just large ones like the eruption of Mt. Pinatubo. Low-level volcanic activity in the past decade likely plays a role in surface warming trends.

Climate models aren’t like a local weather forecast—they actually function better over longer timescales. In other words, the models are good at estimating the cumulative, long-term effects of factors such as eruptions and El Niño events, but aren’t accurate enough to predict when an individual eruption or El Niño event will occur. This means that in the short term, variations still exist that the models can’t be expected to match.

Fyfe et al. shows that the models overestimated warming for the period of 1998-2012, but over the longer timescale of 1900-2012, the models were right on the money. Over the long-term, variations averaged out to reveal the overarching trend. There is no dispute that this trend is toward warming, and the models show us that if emissions continue, warming will continue over the course of the next century.

Models give us a broad picture of scenarios we otherwise would be unable to see. Scientists know that models aren’t perfect and take model uncertainties and shortcomings into account. For example, we know models may be thrown off by random events like volcanic eruptions, and they are limited by the current state of knowledge on topics like deep ocean dynamics. However, it is important to remember the conclusions of climate science are not based completely or even largely on models. But we don’t have another Earth to test, and we don’t want to wait decades to see what the climate will do. By then it will be too late.

Both models and direct observations make one thing clear: the path we are on leads to dangerous warming, and the way to change course is to reduce our emissions.

Relevant Quotes from Scientists:

Dr. Bill Collins, Professor of Atmospheric Chemistry and Earth System Modelling at the University of Reading:

  • “Climate models, which are often criticised, have improved significantly in their detail and complexity since the last report was produced in 2007, as a result of improvements in computing power and more sophisticated ways of representing the climate system. Along with more accurate observations of current and past climate, these models show the crucial benefits of reducing our emissions of CO2 and other pollutants in the future to prevent the worst-case scenarios in the parts of the globe most vulnerable to climate shocks.” (Science Media Centre)

Dr. Tom Wigley, climate scientist at the University Corporation for Atmospheric Research:

  • “[T]he recent slowdown in the rate of global warming, although not yet fully understood, is not an indication of flaws in models or our understanding of the greenhouse effect…Model projections tell us what the expected signal should be, but cannot predict the background noise in which this signal is embedded. All we are seeing today is an unusual manifestation of the noise that has significantly masked the underlying signal.” (Australian Research Council)

Dr. Piers Forster, Professor of Physical Climate Change at the University of Leeds:

  • “[IPCC authors] recognize that the slowdown is an important event to understand, especially as it is not obviously reproduced in climate models. The slowdown since 1998 itself is likely due to a combination of natural (solar and volcanic effects) and extra heat from greenhouse warming being sucked into the deep ocean. Climate models can capture such slowdown events but there is the possibility that some models are over responsive. Any over-response would only be a small effect though and the slowdown does not significantly affect our 2100 projections.” (Science Media Centre)

What the AR5 says:

“The forcing from stratospheric volcanic aerosols can have a large impact on the climate for some years after volcanic eruptions. Several small eruptions have caused a RF of -0.11 [-0.15 to -0.08] W m-2 for the years 2008-2011, which is approximately twice as strong as during the years 1999-2002.” (SPM-9)

“The observed reduction in surface warming trend over the period 1998-2012 as compared to the period 1951-2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend.” (SPM-10)

“There is medium confidence that internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of internal variability.” (SPM-10)

“Understanding recent changes in the climate system results from combining observations, studies of feedback processes, and model simulations. Evaluation of the ability of climate models to simulate recent changes requires consideration of the state of all modeled climate system components at the start of the simulation and the natural and anthropogenic forcing used to drive the models. Compared to AR4, more detailed and longer observations and improved climate models now enable the attribution of a human contribution to detected changes in more climate system components.” (SPM-10)

Useful Visuals:

Climate Model Projections

This figure from the 2013 IPCC report compares the global surface warming projections made in the 1990, 1995, 2001, and 2007 IPCC reports to the temperature measurements. Solid lines and squares represent measured average global surface temperature changes by NASA (blue), NOAA (yellow), and the UK Hadley Centre (green). The colored shading shows the projected range of surface warming in the IPCC First Assessment Report (FAR; yellow), Second (SAR; green), Third (TAR; blue), and Fourth (AR4; red). Source: 2013 IPCC Working Group 1 report.

Act Now: 95% Certainty is as Good as it Gets

How certain are scientists regarding recent warming trends and their human cause? Very. The high degree of certainty within the scientific community—currently pegged at 95%—makes it all the more necessary to act now to reduce our emissions and impact on the climate.

When climate scientists say they are 95% certain that humans are mostly to blame for temperatures increases since 1951, it technically means that there is only a 0-5% chance that random effects produced most of these observed changes. But when this “science speak” is put into context, the statement denotes much more. It signifies overwhelming and undeniable proof that humans are altering the climate system.

Scientists use theories to figure out how the world works, and then test them. Scientists build their theories in accordance with observable facts. They accept nothing as 100% proven, but instead use statistical methods to quantify their specific degree of confidence that study results are not due to chance. This leaves the door open to adjust their theories when new information is discovered.

Even concepts we all take as fact, like gravity, are scientifically classified as theories. A recent Associated Press story highlighted, for example, that scientists aren’t even 100% certain the sun will come up tomorrow.

This article also cited these examples to put 95% certainty into perspective:

  • 95% certainty is equivalent to the current scientific certainty that the universe is 13.8 billion years old.
  • 95% certainty is equivalent to the current scientific certainty that smoking cigarettes is linked to cancer.

This graphic featured in the journal Nature shows the progression of certainty levels about human contribution to climate change in IPCC reports (look at the bottom row: “Gaining Confidence”), as scientists have more and more evidence of human influences on climate change.

Relevant Quotes from Scientists:

Dr. Heidi Cullen, Vice President and Chief Climatologist for Climate Central:

  • “We live in a world where hyperbole feels very commonplace. But within the scientific community that is very unacceptable. For scientists the word ‘likely’ means 66-90% certain, ‘very likely’ means greater than 90% certain, ‘extremely likely’ means 95-100% certain. We (scientists) tend to be far more precise with language, sometimes to our own disadvantage. A statement that there is 95% certainty that climate change is a result of our actions, in and of itself, is quite profound. We’re about as precise and confident as you can get in that.”

Dr. James McCarthy, Professor of Biological Oceanography at Harvard University:

  • “What if a body of comparable authority (to the IPCC), such as the world’s experts on lung disease or heart disease, like the American Cancer Society or the American Lung Association, said that they are 95% certain that smoking will contribute to cancer or lung disease. There would be no doubt to the public what that means. We’re in a similar situation right now.”
  • “We need to make sure the non-scientific community understands how confident the scientists are with these statements. The caution of scientists of never wanting to overstate on this has had us (the IPCC) seeing changes that now upon reflection need a stronger statement, further change, a stronger statement, we’re now at a point where you know we’re within a whisker of being virtually certain.” 

Dr. George Gray, Director of the Center for Risk Science and Public Health at George Washington University, and Chief Scientist for the U.S. Environmental Protection Agency during the George W. Bush administration:

  • “There’s a group of people who seem to think that when scientists say they are uncertain, we shouldn’t do anything. That’s crazy. We’re uncertain and we buy insurance.” (Associated Press)

Dr. Michael Oppenheimer, Climate Scientist at Princeton University:

  • “(Climate change) is not as sure as if you drop a stone it will hit the Earth. It’s not certain, but it’s close.” (Associated Press)

What the AR5 says:

Certainty of human attribution

  • “Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes (Figure SPM.6 and Table SPM.1). This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” (SPM 12)
  • “It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human induced contribution to warming is similar to the observed warming over this period.” (SPM 12)