Author Archives: Ilissa Ocko

How Do We Know That Humans Are Causing Climate Change? These Nine Lines of Evidence

While most Americans acknowledge that climate change is happening, some are still unsure about the causes.

They are often labeled “climate skeptics,” but that label can cause confusion or even anger.

Isn’t the nature of science to be skeptical? Isn’t it good to question everything?

Yes, but —

Here’s what is getting lost in the conversation:

Scientists have been asking these questions for nearly 200 years. The scientific community has been studying these questions for so long that collectively they have amassed an overwhelming amount of evidence pointing to a clear conclusion.

A similar situation is smoking and cancer. Nowadays, no one questions the link between smoking and cancer, because the science was settled in the 1960s after more than 50 years of research. The questions have been asked and answered with indisputable evidence.

We can think of the state of human activities and climate change as no different than smoking and cancer. In fact, we are statistically more confident that humans cause climate change than that smoking causes cancer.

Our confidence comes from the culmination of over a century of research by tens of thousands of scientists at hundreds of institutions in more than a hundred nations.

So what is the evidence?

The research falls into nine independently-studied but physically-related lines of evidence, that build to the overall clear conclusion that humans are the main cause of climate change:

  1. Simple chemistry that when we burn carbon-based materials, carbon dioxide (CO2) is emitted (research beginning in 1900s)
  2. Basic accounting of what we burn, and therefore how much CO2 we emit (data collection beginning in 1970s)
  3. Measuring CO2 in the atmosphere and trapped in ice to find that it is indeed increasing and that the levels are higher than anything we've seen in hundreds of thousands of years  (measurements beginning in 1950s)
  4. Chemical analysis of the atmospheric CO2 that reveals the increase is coming from burning fossil fuels (research beginning in 1950s)
  5. Basic physics that shows us that CO2 absorbs heat (research beginning in 1820s)
  6. Monitoring climate conditions to find that recent warming of the Earth is correlated to and follows rising CO2 emissions (research beginning in 1930s)
  7. Ruling out natural factors that can influence climate like the Sun and ocean cycles (research beginning in 1830s)
  8. Employing computer models to run experiments of natural vs. human-influenced “simulated Earths” (research beginning in 1960s)
  9. Consensus among scientists that consider all previous lines of evidence and make their own conclusions (polling beginning in 1990s)

(You can also see these nine lines of evidence illustrated in the graphic below)

Skeptics sometimes point to the last two supporting lines of evidence as weaknesses. They’re not. But even if you choose to doubt them, it is really the first seven that, combined, point to human activities as the only explanation of rising global temperatures since the Industrial Revolution, and the subsequent climate changes (such as ice melt and sea level rise) that have occurred due to this global warming.

The science is settled, and the sooner we accept this, the sooner we can work together towards addressing the problems caused by climate change – and towards a better future for us all.


(Click here for a pdf version of the graphic)


Posted in Basic Science of Global Warming, Greenhouse Gas Emissions, Science, Setting the Facts Straight| 39 Responses

The Impacts of Climate Change on Human Health – a Sobering New Report

We have even more information this week about the ways climate change poses a threat to human health.

The U.S. Global Change Research Program just released its newest report—The Impacts of Climate Change on Human Health in the United States. This scientific assessment is the culmination of three years of work by hundreds of experts, and builds on the more general National Climate Assessment released in 2014.

The report concludes that every American is vulnerable to the health impacts associated with climate change.

Health Threats from Climate Change graphic

Graphic created by Ilissa Ocko, EDF Scientist

Scientists have known for decades that climate change threatens human health via excessive heat, worsened air quality, water related illnesses, food safety, diseases transmitted by pests like fleas and mosquitos, and mental stress. The new report thoroughly characterizes our current understanding of these impacts.

Because scientific understanding has advanced significantly in recent years, the authors also reviewed new information and insights from several recent scientific, peer-reviewed publications and other publicly available resources.

For example, new data revealed that the Ragweed season has grown by as much as 27 days in the central U.S. from 1995 to 2011, and the incidence of Lyme disease in the Northeast has doubled from 2001 to 2014, both consistent with warming trends.

Recent modeling studies have also improved quantification estimates of and confidence in projected health outcomes from climate change. By midcentury, scientists project that there will be as many as thousands of additional ground-level ozone (smog) related illnesses and premature deaths, and the majority of the western U.S. will have a 500 percent  increase in the number of weeks with risk of very large fires. By the end of the century, scientists project that there will be an additional 27,000 summertime heat-related deaths annually in over 200 U.S. cities (that are currently home to 160 million people), and harmful toxin-producing algal blooms could develop up to two months earlier and persist for up to two months longer.

Through climate and weather changes and disruptions to ecosystems and societal systems, here are the main concerns about climate change impacts on human health:

  • Temperature Related Death and Illness — Future climate warming could cause up to tens of thousands of additional deaths each year from heat in the summer, from loss of ability to control internal temperature, and worsened chronic cardiovascular and respiratory diseases
  • Air Quality Impacts — The future could include limited productivity at work and school due to exacerbated ground-level ozone (smog) health impacts from modified weather patterns conducive to ozone formation, and worsened allergy and asthma conditions from more airborne pollen and longer pollen seasons
  • Vectorborne Disease — The seasonality, distribution, and prevalence of vectorborne diseases, including Lyme disease and West Nile virus,  may change with changing temperature and rainfall patterns due to altered geographic and seasonal distributions of mosquitoes, ticks, and fleas
  • Water-Related Illness — Risk of exposure to illnesses increases as the growth, survival, spread, and toxicity of water-related pathogens and toxins is impacted by temperature and extreme rainfall events, and aging water infrastructure is vulnerable to failure with extreme events and storm surges
  • Food Safety, Nutrition, and Distribution — Rising temperatures, changing weather patterns, and extreme events have consequences for contamination, spoilage, and the disruption of food distribution, whereas higher carbon dioxide levels lower nutritional value of crops despite boosting plant growth
  • Extreme Weather — Fatalities, injuries, and infrastructure damages are imminent with increases in the frequency and/or intensity of extreme precipitation, hurricanes, coastal inundation, drought, and wildfires
  • Mental Health and Well-Being — Mental health conditions may develop with exposure to disasters or worsen by extreme health

Overall, the report is a sobering portrait of the risks we face because of climate change — and it underscores the urgency for climate action.




Posted in Basic Science of Global Warming, Extreme Weather, Health, News, Plants & Animals, Science| Comments are closed

3 reasons the Zika outbreak may be linked to climate change

The regions that the Zika virus outbreak has struck hardest, such as Brazil and Colombia, also happen to be areas that are currently plagued by hotter-than-usual temperatures.

So is there a connection?

The ways that virus-carrying mosquitoes change their behavior with warmer temperatures may, in fact, point to a link between the Zika outbreak and climate change like the one that exists with malaria, Lyme Disease and other ills.

While it’s important to remember that it’s probably a combination of reasons for the current Zika virus outbreak – including movement of people and available breeding grounds – there are three ways in particular that warmer weather may be contributing to the crisis:

graphic_v3 (2)

1. Hotter temperatures make mosquitoes hungrier

Female mosquitoes require blood meals for reproduction. Along with many cold-blooded animals, mosquitoes feed more frequently with higher temperatures. The more they eat, the likelier they are to get infected and spread the disease.

2. Warm air incubates the virus faster

A virus must incubate inside a mosquito before the mosquito becomes infectious. That takes about 10 days, roughly a mosquito’s lifespan, so the mosquito will often die before it can spread the disease.

But hotter temperatures speed up the incubation process in the cold-blooded mosquito, because the virus can replicate faster. This means that the mosquito will be alive longer while infectious, thus having more time to transmit the disease.

3. Mosquito territory expands as the climate warms

Mosquitoes flourish in warm climates, restricting their range based on temperature. But with climate change, plants and animals are moving northward and upward, and we know mosquitoes do the same as new areas become warmer and a suitable habitat.

As mosquitoes expand their range, they can introduce diseases to populations that otherwise would have been safely out of reach. The distribution of the Zika-carrying mosquito, in particular, has wildly increased over the past few decades, which have also been the hottest decade on Earth in more than 1,000 years.

In fact, the current epidemic took off in 2015, the hottest year in South America and globally since record-keeping began 136 years ago.

The links between mosquitoes and temperature are scientifically clear, and it’s possible that climate change may now be playing a role in the spread of the Zika virus, a disease suspected of causing serious birth defects.

To know for sure, and to help nations deal with the outbreak, more research is needed to tease out the specific causes of this global catastrophe.

This post originally appeared on our EDF+Voices blog.

Posted in Health, News, Plants & Animals, Science| Comments are closed

Climate Change and Millennials – An Entire Lifetime of Warmer Than Average Temperatures

While reading the announcement that 2015 had broken – indeed, shattered – the hottest year on record set by 2014, there was one fact that really made things personal: we have now had 31 straight years since a single month was cooler than the twentieth century global average temperature. That means that I have never lived through a month that wasn’t warmer than average – never once in my lifetime.

My entire career as a climate scientist is focused on reducing the threat of global warming, and yet I have never even been alive at a time when the climate was stable. I technically don’t even know what normal is.

Warmest Years on Record graphic

So on one hand, you could say that I don’t even know what I am fighting for. On the other hand, I’ve been afforded two unique opportunities because I’ve lived in the shadow of global warming my entire life.

First, because I’ve grown up at a time when heat records are broken over and over again, I was aware of this worldwide crisis during those impressionable and important “pick a major” years of college. I was thus able to set myself on a career path shaped by climate change from the get-go, rather than later on in life once I was already an established professional in something else.

Second, because my elder colleagues have already identified – with extreme confidence – that humans are the main cause of climate change, I’ve been able to focus on solutions from the get-go, and not just causes and impacts. I have thus benefitted from previous scientific research because I could explore avenues to address climate change, because if humans are the cause, then we are also the solution.

And it’s not just me; there is now an entire generation of young people motivated and empowered to do something about climate change. We – almost the entire millennial generation – have never lived in a world without global warming.

Perhaps for similar reasons to mine (and/or because we think we’re special), my generation has shown a propensity for not just caring about climate change, but doing something about it. Whether on their campuses of their schools or the communities where they live, my generation is showing that they want solutions. In fact, eighty-percent of millennials support cleaner energy in the U.S., regardless of party affiliation.

For this reason among others, I am more hopeful about our future than ever before. Climate change has been impacting my generation our whole lives, but it doesn’t have to stay that way. We didn’t ask for this challenge, but I truly believe we’ll be able to rise up to meet it.

Posted in Basic Science of Global Warming, Extreme Weather, Greenhouse Gas Emissions, News, Science| Read 1 Response

Six Climate Tipping Points: How Worried Should We Be?

One of the biggest fears about climate change is that it may be triggering events that would dramatically alter Earth as we know it.

Known to scientists as “tipping events,” they could contribute to mass extinction of species, dramatic sea level rise, extensive droughts and the transformation of forests into vast grasslands – among other upheavals our stressed world can ill afford.

Here are the top six climate events scientists worry about today.

1. The Arctic sea ice melts

The melting of the Arctic summer ice is considered to be the single greatest threat, and some scientists think we’ve already passed the tipping point.

As sea ice melts and the Arctic warms, dark ocean water is exposed that absorbs more sunlight, thus reinforcing the warming. The transition to an ice-free Arctic summer can occur rapidly – within decades – and this has geopolitical implications, in addition to a whole ecosystem being disrupted.

Photo: Smudge 900)

2. Greenland becomes ice-free

The warming of the Arctic may also render Greenland largely ice-free. While Greenland’s ice loss will likely reach the point of no return within this century, the full transition will take at least a few hundred years.

The impacts of the Greenland ice melt is expected to raise sea levels by up to 20 feet.

Half of the 10 largest cities in the world, including New York City, and one-third of the world’s 30 largest cities are already threatened by this sea level rise. Today, they are home to nearly 1.8 billion people.

Other vulnerable American cities include Miami, Norfolk and Boston.

Photo: siralbertus

3. The West Antarctic ice sheet disintegrates

On the other side of Earth, the West Antarctic ice sheet is also disintegrating. Because the bottom of this glacier is grounded below sea level, it’s vulnerable to rapid break-up, thinning and retreat as warm ocean waters eat away at the ice.

Scientists expect the West Antarctic ice sheet to “tip” this century, and there is evidence that it already began happening in 2014.

However, the entire collapse of the glacier, which would raise sea level by 16 feet, could take a few hundred years.

Photo: BBC World Service

4. El Niño becomes a more permanent climate fixture

The oceans absorb about 90 percent of the extra heat that is being trapped in the Earth system by greenhouse gases. This could affect the ocean dynamics that control El Niño events.

While there are several theories about what could happen in the future, the most likely consequence of ocean heat uptake is that El Niño, a natural climate phenomenon, could become a more permanent part of our climate system.

That would cause extensive drought conditions in Southeast Asia and elsewhere, while some drought-prone areas such as California would get relief.

The transition is expected to be gradual and take around a century to occur – but it could also be triggered sooner.

Photo: Austin Yoder

 5. The Amazon rain forest dies back

Rainfall in the Amazon is threatened by deforestation, a longer dry season, and rising summer temperatures.

At least half of the Amazon rainforest could turn into savannah and grassland, which – once triggered – could happen over just a few decades. This would make it very difficult for the rainforest to reestablish itself and lead to a considerable loss in biodiversity.

However, the reduction of the Amazon ultimately depends on what happens with El Niño, along with future land-use changes from human activities.

Photo: World Bank

 6. Boreal forests are cut in half

Increased water and heat stress are taking a toll on the large forests in Canada, Russia and other parts of the uppermost Northern Hemisphere. So are forest disease and fires.

This could lead to a 50-percent reduction of the boreal forests, and mean they may never be able to recover. Instead, the forest would gradually transition into open woodlands or grasslands over several decades.

This would have a huge impact on the world’s carbon balance because forests can absorb much more carbon than grasslands do. As the forest diminishes, the climate will be affected as will the Earth’s energy balance.

However, the complex interaction between tree physiology, permafrost and fires makes the situation tricky to understand.

Photo: Gord McKenna

Other concerns…

As if that’s not enough, there are a few other tipping events that scientists are also concerned about, but they are even more complex and harder to predict. Examples of such events include the greening of the Sahara and Sahel, the development of an Arctic ozone hole and a chaotic Indian summer monsoon.

How do we keep from tipping over?

We know from measurements that the Earth has had many climate-related tipping events throughout its history. Today’s situation is different, because humans are now driving these changes and the warming is occurring at a faster rate.

But as humans we also have the power to change the trajectory we’re on – possibly in a matter of a few years. We think we know how.

Posted in Arctic & Antarctic, Basic Science of Global Warming, Extreme Weather, Greenhouse Gas Emissions, Oceans, Plants & Animals| Read 1 Response

Just Two Actions May Stop the Planet's Runaway Warming

I was 15 and I was trying to impress a boyfriend with my rollerblading skills — from the top of a steep hill. Before I knew it, I was flying uncontrollably toward traffic. I knew I needed to both slow down and change course . . . or things wouldn't end well.

I did, and I survived, but I've recently thought about that day and those actions as I have considered the urgency needed for the planet to slow down and change course as the climate warms. With two major actions, we can slow the rate of global warming while also preventing "runaway" warming: nations must reduce emissions of both short-lived and long-lived pollutants.

All emissions are not equal

The way people talk and think about the long and short-term impacts of various greenhouse gasses is critical for making smart policy decisions that can effectively slow how fast the climate changes while limiting warming in the future.

While the maximum extent of warming relies on carbon dioxide (CO2) emissions because they last for centuries in the atmosphere, the rate of climate change is controlled by short-lived climate pollutants, such as methane.

Like carbon dioxide, methane is a gas that warms the Earth by trapping heat. Pound for pound, methane is more than 100 times more powerful than CO2 because methane is much more efficient at absorbing heat. But that number changes depending on how far out you look.

Comparing emissions of gases with vastly different radiative impacts and atmospheric lifetimes requires a metric that depends on what timeframe you care about, such as the next decade or next century. One way scientists deal with the temporal differences is by measuring the global warming potential of gases over two time periods: 20 years and 100 years.

Methane is 84 times more effective at trapping heat than CO2 over the first 20 years after they are both emitted, and 28 times more effective over 100 years, because most of the methane breaks down in the first 50 years after it is released due to oxidizing chemical reactions. When discussing what actions to take to reduce methane we must think about methane's potency in both timeframes.

Our best chance of combating climate change

Since the Industrial Revolution, methane in the atmosphere has increased by a whopping 150 percent. While in the same period, CO2 levels have gone up 40 percent. Around one quarter of today's human-caused warming is attributable to emissions of methane, while human-caused CO2 emissions account for around half.

The administration of U.S. President Barack Obama is currently undertaking efforts to reduce emissions of some of the most damaging greenhouse gas emissions  responsible for climate change: methane pollution from oil and gas operations and carbon dioxide from coal-fired power plants. This strategy has prompted questions about which climate pollutant should take priority. But the discussion of whether to cut methane emissions first and carbon dioxide later — or vice versa — is not helpful or necessary. We need a two-pronged strategy to stay safe.

Understanding the urgent need to reduce all types of climate pollution, the Obama administration is expected to move forward with rules to mitigate both methane and carbon dioxide in the next few months. This summer the U.S. Environmental Protection Agency (EPA) is expected to propose the first ever direct regulation of methane emissions from new and modified sources in the oil and gas industry, and finalize its Clean Power Plan to reduce carbon dioxide from coal-fired power plants.

Another agency, the U.S. Bureau of Land Management, is also expected to soon propose important rules to reduce wasteful venting, flaring and leaking of methane associated with the production of oil and natural gas on public lands.

Nations cannot solve the climate crisis and prevent serious impacts without simultaneously reducing both short-lived and long-lived climate pollutants. Reducing CO2 will limit the overall warming the planet will experience generations from now, which will have profound impacts on limiting sea level rise and other dangerous consequences.

Reducing warming caused by methane during our lifetime will also reduce the likelihood of extreme weather events and species extinctions — and, a slower rate also provides more time for societies and ecosystems to adapt to changes.

This post originally appeared on LiveScience.

Posted in Clean Power Plan, Energy, Greenhouse Gas Emissions, Science| Comments are closed
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