Guest essay by Eric Worrall
Despite the unsettling title, the authors are not trying to say Earth will end up like Venus if we don’t mend our wicked ways. But the Forbes article and study models positing an early ocean covered Venus appear to make a lot of assumptions, with very little evidence to guide those assumptions.
Venus was once more Earth-like, but climate change made it uninhabitable
December 14, 2020 12.04am AEDT
Scientist-in-Residence, Earth Sciences, Carleton University (also a professor at Tomsk State University, Russia), Carleton University
We can learn a lot about climate change from Venus, our sister planet. Venus currently has a surface temperature of 450℃ (the temperature of an oven’s self-cleaning cycle) and an atmosphere dominated by carbon dioxide (96 per cent) with a density 90 times that of Earth’s.
Venus is a very strange place, totally uninhabitable, except perhaps in the clouds some 60 kilometres up where the recent discovery of phosphine may suggest floating microbial life. But the surface is totally inhospitable.
However, Venus once likely had an Earth-like climate. According to recent climate modelling, for much of its history Venus had surface temperatures similar to present day Earth. It likely also had oceans, rain, perhaps snow, maybe continents and plate tectonics, and even more speculatively, perhaps even surface life.
Less than one billion years ago, the climate dramatically changed due to a runaway greenhouse effect. It can be speculated that an intensive period of volcanism pumped enough carbon dioxide into the atmosphere to cause this great climate change event that evaporated the oceans and caused the end of the water cycle.
…Read more: https://theconversation.com/venus-was-once-more-earth-like-but-climate-change-made-it-uninhabitable-150445
The abstract of the study;
Was Venus the first habitable world of our solar system?
Present‐day Venus is an inhospitable place with surface temperatures approaching 750 K and an atmosphere 90 times as thick as Earth’s. Billions of years ago the picture may have been very different. We have created a suite of 3‐D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 Gya, present‐day Venus orbital parameters, an ocean volume consistent with current theory, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a prograde rotation period slower than ~16 Earth days, despite an incident solar flux 46–70% higher than Earth receives. At its current rotation period, Venus’s climate could have remained habitable until at least 0.715 Gya. These results demonstrate the role rotation and topography play in understanding the climatic history of Venus‐like exoplanets discovered in the present epoch.Read more: https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2016GL069790
The assumptions behind this modelling exercise appear to be a significant stretch. For example;
Venus was resurfaced by volcanic activity hundreds of millions of years ago [e.g., McKinnon et al., 1997; Kreslavsky et al., 2015], so its topography before that time is unknown. As an estimate with some observational basis, we use modern topographic data from the Venus Magellan mission via the PDS (Planetary Data System) archive (http://pds‐geosciences.wustl.edu/mgn/mgn‐v‐rss‐5‐gravity‐l2‐v1/mg_5201) and fill the resurfaced lowlands with water.Read more: Same link as above
The postulated periodic “resurfacing” Venus experiences is far more violent than any volcanic event ever known to have occurred on Earth. The basis of the resurfacing theory is the lack of impact craters on Venus. The estimated age of impact craters which have been observed suggests the entire surface of Venus was covered in lava or otherwise destroyed in a violent volcanic upheaval around 300 million years ago. It seems a big assumption that the previous surface topography of Venus was anything like the current topography.
What about initial atmospheric conditions? Here the models make another big assumption, that the initial atmospheric conditions of Venus resembled Earth.
… Given the fact that Venus shows substantial N2 in its atmosphere today and has few modern day sources or sinks (unlike Earth), we assume that an ancient Venus could have had a ~1 bar N2 atmosphere (1012.6 mb) in its early history. A modern Earth amount of CO2 and CH4 is also included (400 ppm, 1 ppm), given otherwise poor constraints on these gas concentrations. …Read more: https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2016GL069790
Don’t get me wrong, speculative modelling is obviously an interesting intellectual exercise, and might be useful to explore model boundaries or limits. But I think it would be a big leap to believe that models which struggle to explain the current climate of the Earth can tell us anything meaningful about events which occurred hundreds of millions of years ago on another planet.