Mammatus Clouds

The underside of a cloud is usually relatively flat. Warm, moist air rises and cools until, at a particular temperature and altitude, water vapour condenses into tiny droplets. As these droplets grow and scatter sunlight, they create the familiar opaque appearance of a cloud. Under particular combinations of temperature, moisture, and atmospheric motion, however, pockets of condensed air and water droplets can extend downward from the cloud base. Turbulence and air currents within the cloud can organize these features into hanging pouch-like structures. The result is the formation of mammatus clouds, named for their distinctive rounded, pouch-like appearance. The source notes that they are commonly associated with turbulent atmospheric conditions and can occur near thunderstorms.

Mammatus can form in different parts of the world whenever suitable conditions occur. Their visibility, however, depends strongly on illumination. Side-lighting from the Sun can make the hanging structures especially conspicuous, whereas when the Sun is directly above the cloud their shape may be much harder to distinguish.

The original article describes these formations as relatively uncommon and often short-lived, sometimes remaining clearly visible for only minutes.

The photograph reproduced in the post was taken in 2012 by Canadian photographer Michael Johnson. It shows mammatus clouds that appeared following a thunderstorm over Regina, Saskatchewan, Canada.

Publication of the IPCC Sixth Assessment Report — AR6

The latest report from the Intergovernmental Panel on Climate Change (IPCC)—its Sixth Assessment Report—was released on Monday, August 9, 2021, concluding that the threshold of approximately 1.5°C of global warming can be reached around 2030, roughly a decade earlier than the timeframe which was anticipated several years before.

At roughly 1.2°C of warming, the summer of 2021 has already brought numerous severe weather and climate-related events around the world, including extreme heat in Canada, severe drought in Iran, destructive rainfall and flooding in China, and major wildfires in Greece and California.

The AR6 finds out that atmospheric carbon-dioxide concentrations are higher than at any time in at least two million years, while methane and nitrous-oxide concentrations are higher than at any time in at least 800,000 years.

These findings and the consequences of earlier warnings have not been acted upon. The report indicates that global mean temperature is on course to approach 1.5°C above pre-industrial levels around 2030 under the trajectories being discussed.

Earth Overshoot Day

Yesterday marks Earth Overshoot Day 2021—the point in the year when humanity's demand for ecological resources and services exceeds what Earth can regenerate during that year. In other words, from Overshoot Day onward, humanity is conceptually operating in an ecological deficit for the remainder of the calendar year.

Examining Air-Temperature Anomalies Using Atmospheric Reanalysis Data

A shortage of observational climate data—or limitations in its quality—in different regions can reduce our understanding of climate processes and weaken management capacity in meteorology, hydrology, and agriculture. Today, advances in forecasting centres, climate-data modelling, modern processors, and supercomputers make it possible to predict weather by combining numerical models with data assimilation: the incorporation of direct observations and remotely sensed measurements into model calculations.

What is atmospheric reanalysis?

With sufficient computing resources, advanced numerical models can predict air temperature and other atmospheric variables over short- and medium-range future periods. Increasingly, they can also incorporate near-real-time data—observations assimilated into numerical forecasts soon after they are collected.

Projecting Climate Change

How do scientists project climate change decades into the future—for example, 50 or 100 years from now? Among the most powerful tools for assessing future climate change are coupled atmosphere–ocean general circulation models, commonly referred to as global climate models. These models represent physical processes, atmospheric circulation, and the dynamic behaviour of Earth's atmosphere and oceans.

Future emissions scenarios

Global climate models are run under different scenarios representing possible future greenhouse-gas emissions. These scenarios incorporate assumptions related to factors such as global economic growth, world population, technology, and even social awareness. The figure above illustrates several major scenarios and their corresponding influence on projected changes in global mean annual temperature.

Testing a model before using its projections

Before a global climate model is relied upon for future projections, its skill must be evaluated. The purpose of validation is to determine whether the model can reproduce climate information with an acceptable degree of reliability.

One approach is to run the model for a historical baseline period for which real observations are already available. The model simulates that past period, and researchers then compare statistical characteristics of its output with those of the observed data using appropriate tests and performance measures.

If the simulated and observed statistical characteristics agree sufficiently, the model's skill for the intended application is considered acceptable. Researchers can then run the model using a selected future greenhouse-gas-emissions scenario and evaluate the resulting climatic changes relative to the historical baseline.

The spatial-resolution problem

Global climate models typically simulate meteorological variables on relatively coarse grids. The source gives an approximate spatial scale of 100–300 kilometres. Using such coarse output directly as input to another model—for example, a hydrological model—can increase uncertainty when local or regional detail is required.

To obtain finer-scale information, researchers use downscaling. Downscaling methods are broadly divided into statistical and dynamical approaches. The original article notes that statistical techniques are widely used because they can be faster and simpler to implement than dynamical downscaling.

Climate Change — From Claim to Evidence

Weather and climate conditions are continually changing in different parts of Earth. The natural process known scientifically as climate variability produces variations in the average state of atmospheric variables such as temperature and precipitation across spatial and temporal scales beyond individual weather events. When changes persist and systematically alter the mean characteristics of climatic variables over periods longer than about a decade, they are described as climate change. Climate change can arise from natural processes or human activities. However, researchers attribute the dominant cause of the present era of climate change to human industrial activity.

Image from the original article

Earth's energy budget

Earth and its atmospheric system receive nearly all their energy from the Sun. Under normal conditions, a substantial portion of incoming energy is reflected back to space either before entering deeply into the atmosphere or after interacting with the atmosphere and surface.

Greenhouse gases such as carbon dioxide interact with outgoing infrared radiation, retaining energy within the atmosphere–ocean system and redistributing it through Earth's climate system. The balance among incoming solar energy, outgoing energy, and internal redistribution is commonly described as Earth's energy budget.

The climatological view is that human activity—particularly the release of unprecedented quantities of greenhouse gases—has disturbed this balance by reducing the proportion of energy escaping to space relative to the energy entering the Earth system.

Virtual Water

Professor Tony Allan, the British geographer who developed the influential concept of “virtual water” or “hidden water,” died on April 15, 2021, at the age of 84. He first introduced the concept in 1993 and received the Stockholm Water Prize in 2008 for this work. In one of his important papers, Allan argued that water-scarce regions such as the Middle East could reduce pressure on domestic water supplies through the strategic import of water-intensive products such as wheat and rice from countries with abundant water resources. This idea can be connected with the economic principle of using comparative advantages—including what is called hydrological advantages—while noting that tariffs and restrictions mean global food trade is not completely free and virtual-water trade is consequently more complicated in practice.

What is virtual water?

The hidden water of a product—whether food, a manufactured good, energy, or a service—is the total volume of water used directly and indirectly through the stages of producing and delivering it. For chicken meat, for example, this includes water associated with the bird's feed, growing the grain it eats, slaughter and processing equipment, washing, and other steps.

In other words, virtual water represents the water consumed throughout a product's production chain. An average of approximately 1,300 litres of water is required to grow one kilogram of wheat. “Virtual” does not mean imaginary: most of the water used during production is simply no longer physically contained in the final product. It is real water that has already been consumed.