March 2015 was the warmest March in a 136 years of records, and CO2 levels are now higher than they have been in 800,000 years. If you are an environmental activist, or someone who cares and wants to help, you may find yourself confronting a denialist campaign that sows doubt and confusion. Here is some useful information about the current state of global warming.

Dry Farmland in MongoliaFarmer Zhang Dadi in his dry corn field in Mongolia. Global warming has stricken farmers around the world. © Qiu Bo / Greenpeace.

Doubt: Petroleum interests and paid denialist employ scientific doubt to rationalize non-action, but this is a trick. Scientific knowledge is built on doubt. Every process in nature involves multiple influences, no observer knows all the factors, and everything science knows is framed by a margin of doubt. Nevertheless, science has observed enough to know that global warming is real, and that the primary cause is human activity.

The fundamental hypothesis: In 1896, using known observations of energy radiance and conduction, Swedish chemist Svente Arrhenius introduced the fundamental postulate: "If the quantity of carbonic acid [CO2] increases … the temperature will increase." CO2 in the atmosphere absorbs reflected light, adding heat to the Earth system.

Greenhouse effect: Greenhouses store heat because light changes when reflected. Solar energy enters and passes through a greenhouse glass, or our atmosphere, at "short" wavelengths (0.1 - 4 microns or millionths of a meter). Once reflected, light is polarized and has a longer wavelength (4-50 microns). Carbon dioxide absorbs light at around 15-microns, other gases, such as methane, absorb at other wavelengths, and this absorbed light energy adds heat to the Earth system.

"Global warming" defined: Temperature is always fluctuating, but Climatologists have defined "Global warming" as a relatively large change in a short time, specifically: 0.4°C in one century. Earth's temperature has increased by 0.8°C in one century, a state of global warming. (Goddard)

Weather vs. Climate: Weather is local and short term; climate is regional or global, and long term. A cold winter is weather, and does not indicate the direction of climate change.

Forest and Mine Site. 09/15/2009 © Greenpeace / John WoodsThe Canadian tar sands open-pit mine in Alberta is one of the major contributors to Global warming, releasing carbon to the atmosphere, while removing the Boreal Forest. 09/15/2009 © Greenpeace / John Woods

Do humans contribute to global heating? Yes. We contribute to heating because we produce CO2 and other greenhouse gases in the atmosphere and because we reduce carbon-sequestering plant life. The tars sands in Canada does both, producing vast amounts of carbon while destroying forests.

Human carbon emissions. World carbon emissions through Fossil fuels, cement, land-use change, and other sources were about 1 billion tons / year a century ago. Those emissions are now about 10 billions tons / year (9.9 billion tons in 2013, CO2Now). You may hear of "carbon dioxide emissions" around 35 billions tons, and this is because a ton of carbon produces about three-and-a-half tons of CO2.

Rate of change: These emissions are now about 61% higher than they were in 1990 (the Kyoto Protocol reference year), and still increasing at about 2.5% per year, on track to double in 28 years.

Sources of CO2: Carbon emissions are dominated by China, the US, Europe, and now India. The primary sources are coal, oil, gas, and cement manufacturing. Meanwhile, carbon uptake by plant life is reduced through deforestation and ocean acidification.

CO2 in atmosphere: Before the industrial revolution, some two hundred years ago, atmospheric carbon-dioxide fluctuated around 280 parts per million (ppm). Today, by March 2015, CO2 has reached 401.5 ppm (Scripps), a 43% increase in two centuries.

Rate of CO2 increase has, itself, been accelerating. In the 1950s, atmospheric CO2 was increasing at about 0.5 ppm per year; by 1970, by 1 ppm per year, and is now increasing by 2.1 ppm / year.

Are humans the primary cause of global heating? Yes, this is extremely likely. For anything to heat or cool, a force is necessary, called a "radiative forcing," measured in watts per square meter, (w/m2). Smaller forcings include ozone, water vapour, carbon soot, sulfates, land use changes, Earth's albedo (reflective quality), and reduced ocean CO2 absorption due to acidic water. Intermittent volcanic aerosols have a cooling effect. These smaller forcings somewhat offeset each other. There are three larger forcings, shown here as changes between 1880 and 2011:

1. Human gases: + 3.10 w/m2 (heating)

2. Solar variations: + 0.12 w/m2 (heating)

3. Human aerosols: - 1.60 w/m2 (cooling)

This chart below (James Hansen, NASA) shows the result of adding all these forcings, large and small, heating and cooling: a + 1.5 w/m2 heating effect, primarily caused by human greenhouse gases. We do not know of any forcing greater than the human greenhouse gases, and to this we must add the human reduction in carbon-sequestering ground cover. If anyone believes there is a source of global heating greater than human activity, ask them to state what that forcing is. Of course, unknown factors may exist, but the available information shows us that humans stand out as the primary cause of modern global heating since 1750.

Global climate Forcings.  © James Hansen, NASA

Feedback and runaway: The danger civilization faces is that we can easily lose control of global warming. The heating itself causes feedbacks within the ecological system, which in turn increase heating. These include:

1. Methane from melting permafrost, a more potent greenhouse gas than CO2.

2. Albedo: Ice melt reduces reflection, and increases heat absorption.

3. Water Vapour: Warming adds moisture, a greenhouse gas, to the atmosphere.

4. Forest loss: Each year, we lose about 15 million hectares of forests.

5. Acidic seas: reduces aquatic life and carbon capture.

6. Fires: A hotter climate increases fires that release CO2 and reduce forest cover.

California's Lake Oroville. © Justin Sullivan, Getty Images

The effects: NASA, the UN, and scientific agencies around the world have observed and reported on the effects of global warming. The picture above (Justin Sullivan, Getty Images) shows the effects of drought on California's Lake Oroville. Here are some of the observed effects of global warming:

Heat: Earth's average temperature has increased by 0.8°C in one century.

Arctic: average temperature increase is about twice the global average.

Ocean temperature has increased to depths of 3,000 meters.

Rate of warming has nearly doubled in the last 100 years.

Warmest years: Of the last 12 years, 11 rank among the warmest since 1850.

Ice melt: Glaciers and polar ice melting in Northern and Southern hemispheres.

Sea level has risen about 20 cm in a century, and the rate of rise is increasing.

Extreme weather: more intense tropical storms, heat waves, drought.

Precipitation has increased in eastern Americas, northern Europe, and Asia.

Drying and drought in Southwest US, Mexico, Mediterranean, southern Africa.

Species: Diversity loss due to climate changes and habitat destruction.

Agriculture disruptions, such as reduced yields from warmer and wetter climates.

Ocean Acidification: The oceans are about 30% more acidic compared to the pre-industrial era, killing off sea life and reducing vital coral reef ecosystems.

Historic Climate Change: Denialists use past fluctuations to proclaim that modern warming is not caused by human activity, cherry-picking isolated data to misrepresent global warming. Here is a brief history of Earth's changing climate:

Young Earth: Earth formed 4.5 billion years ago as molten rock, and cooled over the next 3 billion years. Volcanoes released gases: hydrogen, carbon dioxide, sulfates, and nitrogen. Water condensed, bacteria formed, and photosynthesis produced oxygen, which poisoned bacteria, the first major extinction. This comprised about half of Earth's history.

First warming: About 2 billion years ago, some bacteria learned to breath oxygen; released CO2, and Earth heated up, the first global warming, and new extinctions.

Plant boom, 550-470 million years ago (mya): As CO2 increased and oxygen levels dropped, plant life recovered, captured carbon, and Earth cooled. Ninety percent of Earth's history had passed.

Animal boom, 450-350 mya: The plant die-off released CO2; fish, amphibians, and reptiles released more CO2; and Earth warmed.

Land plants, 385 - 265 mya: The boom in land plants captured carbon, and Earth cooled. This boom of life created the hydrocarbons, oil and coal, that we now burn.

Land animals, 265-65mya: Again, the plant die-off and animal boom released CO2, and Earth warmed. By 100mya, CO2 content reached 2,000 ppm, and the average temperature was about 11°C hotter than today.

Ice ages: 65 million years ago, an asteroid hit Earth near Yucatan, Mexico. Earth has generally cooled since then, punctuated by warming fluctuations and ice ages. Forests captured carbon, and humans evolved. By the time humans controlled fire, about 99.9% of Earth history had passed.

Temperature and CO2, over 400,000 years. © M. Ernst, Woods Hole Research Center

Modern warming: The chart above (ice core data, M. Ernst, Woods Hole Research Center), shows that for the last 400,000 years, Earth's temperature (brown) and CO2 levels (blue) have fluctuated in lock-step, CO2 levels between 200-300 ppm, and temperature between 9°C cooler and 3°C warmer than today. About 3,200 years ago, CO2 and temperature spiked, causing worldwide flooding as recorded in human cultural stories. About 250 years ago, industrial advancement increased coal and oil use, releasing CO2, and heating Earth. CO2 levels have now reached 400ppm, and temperatures have risen almost 1°C. The data suggests that Earth may be headed for severe temperature increases, due to this CO2 build up in the atmosphere.

The Future: If humans act wisely — if we reduce consumption, stabilize population, and abandon hydrocarbon energy — we could reverse the modern warming that we have set in motion. If we fail, we face runaway heating.

A 2009 MIT study estimated that there is now a 90% chance that by 2100, CO2 levels will reach 550ppm and Earth's temperature will reach 5.2°C above pre-industrial temperatures.

At those temperatures, melting permafrost will release enough methane to send Earth into a Mesozoic-scale heating, as Earth experienced a hundred-million years ago. Organisms could live in that environment, but humans would have a difficult time, to say the least. Sea rise will wipe out thousands of cities and displace billions of people. Few really want to face this. I do not enjoy writing about it. Avoidance, denial, despair, and anger are completely natural reactions.

Nevertheless, to avoid these outcomes, caring citizens must speak up and help inspire the large-scale and realistic actions that will reverse carbon release into Earth's atmosphere and halt the warming trend.

Rex Weyler is an author, journalist and co-founder of Greenpeace International.

Additional sources:

Human greenhouse gas forcing: David Biello, in Scientific American, Nov. 30, 2009:; Ad Hoc Study Group on Carbon Dioxide and Climate (1979): "Carbon Dioxide and Climate: A Scientific Assessment." US National Academy of Sciences: V. Ramanathan, M.S. Lian, and R.D. Cess (1979): "Increased Atmospheric CO2 .. Radiative Energy Balance and Surface Temperature."

Agriculture disruptions, Example: coffee yields in Columbia; NY Times (March 9, 2011.

Global Temperature: Columbia University: Global Temperature and Temperature 2014-15

Sea Rise: By 2100: 0.5 - 1.2 meters: IPPC 2013; Changing rate: Columbia Univ.

Runaway warming: Rapid, non-linear change: R. Jones, Victoria Univ.

       Abrupt Non-Linear Climate Change: S. Schneider, OECD

Ocean Acidification: InterAcademy Panel, 105 science academies recommended CO2 emissions reduced by 50% from 1990 levels by 2050.

Recent News Articles:

Heat Wave Deaths in India

New Arctic Ice Mass Destabilized

California Redwoods Stressed by Drought

Insurance Company Divests Coal Due to Global Warming