Hummingbirds might be instantly recognizable from the hum (嗡嗡声), but the cause of the characteristic has long been a mystery. Now researchers say they have cracked the conundrum, finally taking the “hmm?” out of hummingbirds.

Scientists carried out experiments with a species known as Anna’s hummingbird. In the setup, scientists arranged microphones and high-speed cameras around a cage in which six hummingbirds fed on an artificial flower, one at a time. This allowed them to pick up the sounds produced by the birds to create a 3D acoustic (声学的) map, which was linked visually with the movement of the wings to explore what was driving the sounds, the team sought to measure the lift and drag forces produced by the moving of the wings in another experiment. This picked up the size of the pressure forces produced and how they changed over time.

When the researchers put the information about the forces together with the motion of the birds’ wings, they were able to predict the sounds that would be created from these factors alone. They then compared them with the 3D acoustic map produced from the microphone setup. The results reveal that aerodynamic (空气动力学的) forces produced as the wings move, together with the speed and direction of the wing movements, are enough to explain the hummingbirds’ hum.

The humming sound is caused by the pressure difference between the upper side and the lower side of the wing, which changes in size and direction as the wing flaps back and forth, the researchers explained. “This is why birds and insects make different sounds,” said David Lentink. co-author of the study. “Most birds are relatively quiet because they only produce their lift once in their downward wing beat. Hummingbirds and insects are noisy because they produce two for every beat of their wings.”

The researchers said that while the findings were interesting in themselves, they could also have applications in future technologies, such as new drones (无人机) that are quieter when flying through the air. Of course, with some hummingbirds flapping their wings up to 80 times a second, it’s unlikely that we’ll see drones capable of copying these unique birds anytime soon.

Earth’s geologic ages---time periods defined by evidence in rock layers----typically last more than three million years. We’re barely 11,500 years into the current age ,the Holocene. But a new paper argues that we’ve already entered a new one—the Anthropocene, or “new man”, age.

The name isn’t brand-new. Nobel Prize winner Paul Crutzen, a co-author of the paper, coined it in 2002 to reflect the changes since the industrial revolution. The paper, however , is part of new push to formalize the Anthropocene age.

Recent human impacts have been so great that they’ll result in an obvious boundary (界限 ) in Earth’s rock layer, the author’s say. “We are so skilled at using energy and exploiting the environment that we are now a defining force in the geological process on the surface of the Earth,” said co-author Jan Zala, a geologist with the University of Leicester in the UK. Even so, it could take years or even decades for the International Union of Geological Science to formalize the new age.

If the concept of the Anthropocene age is to be formalized , scientists will first have to identify and define a boundary line ,or marker, that’s set in stone. “The key thing is thinking about how—thousands of years in the future---geologist might come back and actually recognize in the deposit in the UK.” It’s not as straightforward as you might think. The market has to be very precise, and it has to be recognized in many different parts of the world,” said Haywood, who wasn’t involved in the new study.

One candidate for the market is the distinctive radioactive signature left by atom bomb tests, which began in 1945. “The fallout (沉降 ) is basically across the world,” Haywood said. In a similar way, scientists used traces of the element iridium (铱) left by shooting star strikes to help define the boundary between the Cretaceous and Tertiary periods---the time of the great dinosaur extinctions.

The push for a formal declaration of the Anthropocene age is about more than just scientific curiosity. The move the scientists write in the last issue of the journal Environmental Science & Technology, “might be used as encouragement to slow carbon emissions and biodiversity(生物多样性)loss” or “ as evidence on protection measures” Just as Haywood said, by underlining how much we're changing the environment, the formalization would be "a very powerful statement”.

Seen from the Earth, floating above us, clouds are the stuff of fairy tales, having inspired humans for centuries. But what exactly are clouds and why do they play such a vital role in our survival?

You need the right conditions and just two components to make a cloud: water vapour (蒸汽), and aerosols — tiny airborne particles (微粒) that act as seeds for cloud droplets. These particles come from sources such as dust, volcanoes and salt from sea spray, as well as from human activity, such as ash from burning fossil fuels.

It’s believed that clouds cover about two-thirds of the planet at any given moment and have a big influence on climate and weather. They do this by regulating temperature — for instance, by helping to spread the sun’s energy evenly over the Earth’s surface through storms, which transport heat from warm areas near the equator to colder regions. Clouds also have a thermostat (恒温调节) function, both keeping the Earth cool by reflecting solar energy back into space during the day, and heating it up by trapping thermal energy that rises from the Earth’s surface at night.

Climate scientists have recently turned their attention to this effect when modelling the impact of greenhouse gases on the Earth. Angeline Pendergrass, assistant professor at the Department of Earth and Atmospheric Sciences, tells US news platform, Vox: “How clouds change determines how warm it gets in response to a certain amount of greenhouse gas forcing.” It should be noted, however, that the influence of clouds on climate change is still a topic that’s much debated among scientists. This is because their effects are difficult to model accurately.

According to Angeline, greater accuracy might only be achieved after “large amounts of global warming” have occurred. In the meantime, scientists are paying close attention and will continue to carefully piece together records from the past and observations from the present to project models of various possible futures.