If you are feeling concerned about climate change, perhaps the most important thing you can do is to inform yourself about how human activity has affected the planet so far and what the future looks like if we don’t stop emitting greenhouse gases into the atmosphere.  That should lead you to read the reports produced by the IPCC.

The IPCC, or Intergovernmental Panel on Climate Change, is a scientific body established by the United Nations in 1988 to assess the science, risks, and potential impacts of climate change caused by human activities, and to formulate strategies for mitigating and adapting to its effects. It is composed of thousands of scientists from around the world who volunteer their time to review and synthesize the latest scientific research on climate change.

The panel is primarily organized into three working groups:

• Working Group I focuses on the physical science of climate change;
• Working Group II addresses the impacts, vulnerability, and adaptation to climate change;
• Working Group III studies the options for reducing greenhouse gas emissions and adapting to climate change impacts.

In 2021, Working Group I produced the latest summary of what is now known about the physical science basis of climate change.  The report summarizes what is known about how the climate has changed, provides forecast scenarios of how it could change in the future, and discusses what must be done to mitigate the effects of climate change.

Here are some of the key highlights from that report:

Overall State of the Climate: In 2021 the IPCC concluded that it is unequivocal that human influence has warmed the atmosphere, ocean, and land and that widespread and rapid changes in the atmosphere, ocean, cryosphere, and biosphere have occurred.  The scope of change is unprecedented in human history – which is scary because we are talking about major changes to a fragile balance of climate conditions that support our very existence.

Changes to the atmosphere: As of 2019, the data cut off for the report, the concentration of CO2 in the atmosphere had reached 410 parts per million (ppm) – a level higher than at any time in the last 2 million years. Levels of other greenhouse gases (GHGs) such as methane (CH4) and nitrous oxide (N2O) have also increased dramatically since the 19th century.

Increased concentrations of greenhouse gasses in the atmosphere lead to global warming through a process known as positive radiative forcing: the changes to the atmosphere mean the Earth traps more incoming energy from sunlight than it radiates to space. This net gain of energy causes warming. By calculating a factor called the equilibrium climate sensitivity, scientists are able to estimate how much the planet will warm in response to increasing levels of GHGs.

In their report, the IPCC says that the equilibrium climate sensitivity is now estimated to be 3.0˚C – that is the increase in global average temperature that we would experience for a doubling of CO2 in the atmosphere compared to pre-industrial levels. “Pre-industrial levels” means since 1850. Back then it was ~ 260 ppm of CO2, so a doubling would be 520 ppm.

Temperature findings: Each of the last four decades has been successively warmer than any prior decade since 1850.  Specifically, the IPCC reports that the average global temperature in the 10 years ending 2022 was 1.09˚C higher than this pre-industrial baseline. The warming that we have experienced already is unprecedented in the last 2000 years and is fully attributable to human influence on the climate.

It is important to recognize that these changes are happening now and at an accelerating rate – this has not been a gradual growth in temperature over a long period of time – the temperature has increased faster since 1970 than in any other period in the last 2,000 years.  While a lot of scientific reporting focuses on global averages, this also means that hot extremes have become more frequent and intense since 1950.  Cold extremes are going in the opposite direction.

It was surprising to me to learn that air pollution, aerosols in technical terms, has a cooling effect that helps offset global warming somewhat. Aerosols in the atmosphere, primarily sulfur dioxide, help prevent some solar radiation from getting through the atmosphere in the first place. The IPCC estimates that greenhouse gasses on their own have warmed the planet by 1.5˚C so far, but other aerosols have had a cooling effect of -0.4˚C.

Among the greenhouse gasses themselves, CO2 accounts for about 0.75˚C or about half of the GHG warming observed so far. Methane accounts for about 0.5˚C – about a third.

Precipitation: The findings are that overall levels of general precipitation as well as the frequency and intensity of heavy precipitation events have increased since the 1950s – with a faster rate of increase since the 1980s.  Storm tracks are shifting towards the poles, traveling further north and south than they have previously.  And while it is raining more, changes in how rainfall is distributed – where it falls and how it falls – have also changed, leading to increased droughts in many regions.  That has major implications for the production of food, access to drinking water, and the survival of plant and animal species.

Scientists can’t conclude that there are more hurricanes, but the global proportion of major destructive storms – category 3 to 5 tropical cyclones  – has increased over the last 40 years.

Glacial loss: For the first time in over 2,000 years, glaciers are retreating in every part of the globe.   The Greenland ice sheet is melting as is the sea ice in the Arctic.  The arctic sea ice in summer is now smaller than it has been in 1,000 years.    Across the northern hemisphere scientist are observing an overall decrease in spring snow cover – a measure of the amount of snow that accumulates over the winter.

The decline of glaciers, sea ice, and the snowpack have major implications for the access to freshwater for billions of people and contribute to rising sea levels and coastal erosion. On top of that, we count on the ice and snow to reflect solar radiation back into space, a process called the albedo effect. A less reflective Earth warms even faster.

Ocean temperature and acidity: As a result of CO2 concentrations in the atmosphere it is virtually certain that globally, the upper ocean (the first 700m of depth) has warmed and has reached a level of acidity not seen for over 2 million years.   The rate of warming has not been seen since the end of the last ice age, 11,000 years ago.    This also means that the oxygen content of the oceans is dropping.

The changes in the ocean are causing a wide range of effects on marine life, including reducing the quality and quantity of suitable habitat, reducing growth rates, changing visual function, interfering with reproduction, and increasing disease susceptibility

Sea levels: Sea levels have risen, everywhere, by 20 cm between 1901 and 2018 – an average increase of 1.7 mm per year.  That rate is increasing rapidly – between 2000 and 2018 sea levels rose on average by 3.7mm per year – a rate that is nearly 3x times faster than it had been for most of the 20th century.

Sea levels are increasing for a few reasons.  Half of the increase recorded is because of thermal expansion – as the water in the ocean warms, it takes up more space.   Most of the rest of the increase is explained by melting ice either from glaciers (22%) or sea ice (20%).  The remaining 8% is from water that was previously in the soil returning to the sea through increased evapotranspiration.

Sea level rise poses a serious threat to coastal life around the world. Consequences include increased intensity of storm surges, flooding, and damage to coastal areas. In many cases, coastal areas are where large population centers are located, in addition to fragile wildlife habitats.

Biosphere Changes: The effect of changing climate has affected global growing zones – the various geographic areas with climatic conditions relevant to the growth and survival of different types of plants.  Overall, growing zones are moving – shifting towards the poles as the planet gets hotter.  Growing seasons have also increased in the northern hemisphere – about 2 days per decade since 1950.  While that may sound good, instability about where and when we grow our food has major implications for the food supply for all beings on Earth.

Recognize that climate change has already occurred. The 1.1˚C increase in average temperatures has produced changes in our environment that will affect humanity for centuries to millennia. Recognize too that what we are experiencing today is only a small preview of what the future may hold if we don’t reduce emissions and meet the goals of the Paris Agreement and limit warming to 1.5˚C.

In their reporting, the IPCC has laid out a number of future emission scenarios – from successfully meeting emission targets to a world where we continue to emit GHGs unabated. I’ll summarize those in my next post. ■