Science

Accounting for scientific uncertainty in a dangerously warming world

Most people at some point develop a “Plan B” – in case their first choice of college doesn’t accept them, or it rains on the day of their planned outdoor party, or the deal for the house they wanted falls apart. The same principle applies for more dire situations, such as a city having plans in hand for an orderly evacuation in case of a large-scale disaster. We hope such an event will never happen, but the mayor had better be prepared in case it does.

In a commentary today in the scientific journal Nature Climate Change, three colleagues and I discuss the need for a “Plan B” for climate change: How will we cope with increasingly severe climate impacts if we are unsuccessful in limiting global warming to a chosen target?

In the 2009 Copenhagen climate accord, countries set a goal of limiting global warming to below 2 °C (3.6 °F) above the average global temperature of pre-industrial times. However, given that the planet has already warmed by 0.8 °C, additional warming is already locked into the system, and global greenhouse gas emissions continue to rise, this “Plan A” has become increasingly difficult and may become impossible to achieve if widespread emissions reductions do not begin within this decade. A maximum warming target is a necessary goal of climate policy, but what if our efforts fall short?

Some voices in the environmental community will feel that asking this question is ceding failure, but I disagree. Instead, it means admitting that we can’t perfectly foresee the future and that we need to be prepared for surprises. This is called risk management and everyone from parents, to mayors, to companies, to the U.S. military uses risk management every day to cope with uncertainty.

The State of the Climate

As President Barack Obama prepares to deliver his State of the Union address, we believe it’s a good time to take a look at the state of our climate: the growing impacts of climate change, recent progress in reducing U.S. emissions, and further steps we can take to protect the climate and ourselves.

The consequences of rising emissions are serious. The U.S. average temperature has increased by about 1.5°F since 1895 with 80 percent of this increase occurring since 1980, according to the draft National Climate Assessment. Greenhouse gases could raise temperatures 2° to 4°F in most areas of the United States over the next few decades, bringing significant changes to local climates and ecosystems.

Chilling out on the Polar Vortex

This week’s brief but bitter cold snap over more than half the country prompted intense discussion about the polar vortex ranging from educational to bombastic.

Figure 1: A depiction of the “average” polar vortex on Jan. 6. The winds of the vortex correspond to the narrow “rainbow” areas. The map is an average of the upper atmosphere’s “topography” (specifically, the 500 millibar height) from all the January 6ths between 1980 and 2010.
Figure 2: The polar vortex on Jan. 6, 2014. The ridge (“R”) and trough (“T”) responsible for relatively warm weather in much of the West and bitterly cold weather in the Midwest and East have been labeled.

So let’s be clear: The cold snap this week was unusual but not entirely unprecedented. A few super-cold days don’t disprove global warming, just like a day of rain doesn’t end a drought. At the same time, we don’t yet know whether climate change will change the odds of future outbreaks of bitter cold. Research is still underway, and as of now, we shouldn’t necessarily expect these events to be more or less frequent in future winters. 
Here’s a Q&A to cut through the hype:

  • What is the polar vortex?  The polar vortex describes the air circulating aloft (thousands of feet above the ground) about the North Pole, and its extent is marked by a ribbon of strong winds that is often called the “jet stream.” (We most commonly focus on the North Pole, but a similar circulation is present around the South Pole, too).
    In the map (Figure 1), which is from the point of view of the North Pole, the vortex corresponds to purple and blue colored areas. The band where the colors change from blue/purple to red/yellow indicates the location of the jet stream, or the outer edge of the vortex. Winds are strongest where this color gradient is tightly packed (e.g., over the Pacific Ocean and North Atlantic Ocean). It tends to be quite cold at the surface below the purple areas, and warmer under the red/yellow areas.

It’s important to note that this figure is an average of many winter days. On any given day, we would see a number of deviations from this average pattern.

  • What happened this week? Comparing this week (Figure 2) to the average picture (Figure 1), we can see that the purple area of the vortex has contorted and moved farther south. Along with this pattern, there are substantial “wiggles” in the jet stream. These deviations in the circulation helped bring cold air into the continental United States that normally stays in northern Canada and the Arctic. Meteorologists look for these wiggles, called “ridges” and “troughs” (“R” and “T” on the map)  when putting together a forecast. While the trough brought notable cold to the Midwest and the East, the ridge has kept parts of the West warmer than average and relatively dry (much to the dismay of skiers).

Four ways climate change is transforming our winters

Joe Casola, staff scientist and program director of Science and Impacts, and Dan Huber, science and policy fellow, co-authored this article.

The terms “climate change” and “global warming” might conjure up images of balmy beaches and scalding deserts – a world without winter. But it’s more complicated than that.

As we prepare for the official arrival of the season on Dec. 21, let’s look at a few ways winters in the United States are changing because of global warming, and what we can do to adapt.

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Sandy Anniversary is a Reminder of the Need for Better Protections

A year after Hurricane Sandy, more work remains to be done to help families and communities fully recover. But another pressing need, not only for those who were in Sandy’s wake but for all of us, is to learn from the storm’s devastating impacts and reduce the risk of future damage and loss of life.

Hurricane Sandy's estimated $65 billion in damages make it the second costliest hurricane in U.S. history, surpassed only by Hurricane Katrina.

Building resilience to the impacts of major coastal storms like Sandy—and to other types of extreme weather that are becoming more intense and frequent as a result of climate change—will require a commitment to better protect infrastructure and implement  policies to help get people out of harm’s way. Both efforts should take into account how future sea level rise can amplify storm surges, potentially making future impacts greater than what we’ve experienced in the past.

Proud of what we've done, but there's still more to accomplish

When I founded a new nonprofit organization 15 years ago, the United States and the world urgently needed practical solutions to our energy and climate challenges. That need has only grown more urgent.

Earlier today, I announced my plans to step aside as the President of the Center for Climate and Energy Solutions (C2ES) once my successor is on board. As I look back, I find we have come a long way. That said, any honest assessment of our progress to date in addressing one of this century’s paramount challenges must conclude that we have much, much further to go.

When our organization, then named the Pew Center for Global Climate Change, first launched in 1998, 63 percent of the world’s electricity generation came from fossil fuels. Incredibly, that number is even higher today – 67 percent. The concentration of carbon dioxide in the atmosphere, the main driver of climate change, is also higher than it was then – in fact, at its highest level in more than 2 million years.

Scientists around the globe have just reaffirmed with greater certainty than ever that human activity is warming the planet and threatening to irreversibly alter our climate. Climate change is no longer a future possibility. It is a here-and-now reality. It’s leading to more frequent and intense heat waves, higher sea levels, and more severe droughts, wildfires, and downpours.

We at C2ES have believed from the start that the most effective, efficient way to reduce greenhouse gas emissions and spur the innovation needed to achieve a low-carbon economy is to put a price on carbon. It’s a path that a growing number of countries, states, and even cities are taking.

IPCC AR5 Working Group I Highlights

The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) brings policymakers and the public up to date on the state of climate science. The IPCC report, released in stages, is the most comprehensive assessment of existing climate change research and provides a baseline for understanding and action. The Working Group I Summary for Policymakers released Sept. 27, 2013, states with greater certainty than ever that climate change is happening and that human activity is the principal cause. Among the highlights of the report:

  • The conclusion that much of the warming over the past 50 years is due to human activities is now “extremely likely,” upgraded from “very likely” in the last report.
  • Estimates of future sea level rise have been significantly increased due to a better understanding of the movement of ice sheets in a warming climate.
  • The Arctic Ocean is now projected to be ice-free during the summer by mid-century under a high emissions scenario, instead of the end of century as in previous reports.

Human Activity

Each IPCC report has been progressively stronger in attributing climate change to human activities. The AR5 contains the strongest statement yet, saying it is “extremely likely” (a greater than 95 percent chance) that human activities are “the dominant cause of the observed warming” since the 1950s. The Third Assessment (2001) made a similar statement with approximately 66 percent certainty, while the Fourth Assessment Report (AR4) (2007) found that “most of the observed increase in global average temperatures since the mid-20th century is very likely (greater than 90 percent chance) due to the observed increase in anthropogenic greenhouse gas concentrations.”

Global Changes

The AR4 concluded that “warming of the climate system is unequivocal.” The AR5 goes further, concluding that many observed changes (warming of the atmosphere and ocean, sea level rise and melting ice) are “unprecedented over decades to millennia.”

New atmospheric temperature measurements in the AR5 show an estimated warming of 0.85 degrees Celsius (1.5 degrees Fahrenheit) since 1880 with the fastest rate of warming in the Arctic. The AR4 estimated the average warming across the globe over the past century (1906-2005) was 0.74 C (1.33 F).

Sea Level Rise

The AR5 report has significantly increased projected sea level rise over the next century, due to new research that improves understanding of ice sheet movement and melting. The new projections show an increase of 0.26-0.55 meters (10-22 inches) by 2100 under a low emissions scenario and 0.52-0.98 meters (20-39 inches) under the high emissions scenario. The AR4 did not include some of the effects of ice sheet movement due to warming, and therefore published much lower estimates in the range of 0.18-0.38 meters (7-15 inches) under a low emissions scenario and 0.26-0.59 meters (10-23 inches) under a high emissions scenario for sea level rise by 2100.

Sea and Land Ice

The AR5 projects it is likely (greater than 66 percent chance) that the Arctic Ocean will be ice-free during part of the summer before 2050 under a high emissions scenario. This represents a large shift from the AR4, which estimated that the Arctic Ocean would not be ice-free during the summer until late in the 21st century. The AR5 finds that Arctic sea ice surface extent has decreased by 3.5-4.1 percent per decade (9.4-13.6 percent during summer), which is higher than the AR4 estimate of 2.1-3.3 percent per decade (5-9.8 percent during summer). That amounts to between 0.45 and 0.51 million square kilometers (0.17 to 0.2 million square miles) per decade. The AR5 finds these changes unprecedented in at least the last 1450 years.

The AR5 also states that scientists have “high confidence” (80 percent chance) that glaciers have shrunk worldwide, and that the Greenland and Antarctic Ice Sheets have lost mass over the past two decades.  The report notes with “very high confidence” (90 percent chance) that ice loss from Greenland has accelerated during the past two decades. Greenland is now losing about 215 gigatonnes (Gt) per year of ice, while the rest of the world’s glaciers lose about 226 Gt per year. For comparison, 200 Gt weighs the same as around 100 billion cars (about 1 billion cars exist on Earth today).

Surface Warming “Pause”

After a period of rapid warming during the 1990s, global mean surface temperatures have not warmed as rapidly over the past decade. The AR5 notes there are “differences between simulated and observed trends over periods as short as 10-15 years (e.g., 1998-2012)”. It concludes that the recent reduction in surface warming is probably due to a redistribution of heat in the ocean, volcanic eruptions, and the recent minimum in the 11-year solar cycle.  Most importantly, the report specifically points out that these trends should not undermine our confidence in the “big picture” of our understanding of climate change: “trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”

In addition, there is new research proposing explanations for the recent trends that did not make the deadline to be included in the AR5. One paper suggests that some of this “lost” heat is actually in the deep ocean, while another notes that the warming “pause” is actually explained by the unusual number of La Niña (sea surface cooling events) in the Pacific Ocean. The second paper by Yu Kosaka and Shang-Ping Xie states that the “current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue.”

Cumulative Carbon Budgets

The AR5 relates different carbon “budgets” – an accumulated amount of carbon emissions over time — to the chances of average warming exceeding 2 degrees above 1861-1880 levels. Governments have set an international goal of limiting average warming to 2 C. For the world to have a 50 percent chance of staying below 2 C of warming by 2100, the AR5 identifies a greenhouse gas emissions budget of 840Gt of carbon. More than half of that (over 531GtC) has already been emitted. At current emission rates (around 10 GtC per year), we will use up our carbon budget in just 30 years.

Future Emission Scenarios

The report describes several alternative scenarios of 21st century greenhouse gas concentrations and global temperatures, each associated with different cumulative carbon budgets. Three scenarios represent potential pathways with less warming under various forms of mitigation policy. The fourth represents more of a business-as-usual case, with emissions in the 21st century three to four times larger than the emissions before the 20th century and the highest level of warming in any scenario.

Global surface temperature increases exceed 1.5 C and keep rising beyond 2100 in all scenarios except the lowest-emission scenario, in which actions are taken to nearly eliminate CO2 emissions in the second half of the 21st century. In the scenarios with higher rates of emissions, warming is likely to exceed 2 C by 2100, and could even exceed 4 C.

Getting clear on climate facts

With the Intergovernmental Panel on Climate Change (IPCC) poised to release its Fifth Assessment of the science underpinning our understanding of climate change, it’s useful to take a step back and recap some of the “big picture” facts.

What is already clear from the science:

  • Carbon dioxide and other greenhouse gases act to warm the planet.
  • Carbon dioxide is accumulating in the atmosphere due to emissions from human activities.
  • The Earth has been warming during the past century. The amount and speed of the warming is unusual compared to past records.
  • Humans’ emissions of greenhouse gases are largely responsible for this warming.
  • If emission rates continue, the warming in the 21st century will be much more significant than the warming in the previous century.

IPCC: Growing Certainty on the Human Role in Climate Change

In its periodic assessments, the Intergovernmental Panel on Climate Change has expressed growing certainty that global warming is underway and that human activity is a principal cause. The panel’s language has become progressively stronger over time to reflect its growing certainty.

In 1990, the IPCC said that emissions from human activities were “substantially increasing” greenhouse gas concentrations in the atmosphere, which would lead to warming.

By 2013, the panel had concluded that "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."

The chart below traces how the IPCC’s conclusions have strengthened over time.

YearAssessmentStatement on Human Involvement in Climate Change
1990 First Assessment

“…emissions resulting from human activities are substantially increasing atmospheric concentrations of greenhouse gases…

These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earth’s surface”

1995Second Assessment

“Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin…

…the balance of evidence suggests that there is a discernible human influence on global climate.

…the average rate of warming [in projections for the 21st century] would probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes would include considerable natural variability.”

2001Third Assessment

“There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.

…the projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years, based on paleoclimate data.”

2007Fourth Assessment

“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

2013Fifth Assessment

"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.

…It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."

 

 

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