The Detectability of Planetary Atmospheres and the Fermi Paradox

 It’s often been suggested that the first indication of technological life on Earth observable from space would be the program “I Love Lucy”, which was among the first popular broadcast television shows beginning in 1951.  At this point in time, the sphere of “I Love Lucy” has a radius of 74 light years, and would be detectable to a technological civilization on any of about 4000 star systems.  

It recently occurred to me that over geologic time, many, many, many more star systems than we can detect would have been able to see Earth as a planet with robust life, and the potential to evolve technological intelligence.  Let’s first consider what we can observe in nearby star systems using our current level of technology.

The first discovery of a planet orbiting a main-sequence star occurred in 1995.  Since then, using a variety of improving methods, we have discovered nearly 6000 exoplanets.  The bulk of these tend to be large planets like our gas giants, because of greater ease of detection.  Of these, only 60 or 70 are interpreted to be earth-like, in size, rocky composition, and within the habitable zone of their stars.  
                                                Transiting Exoplanet Survey Satellite

We can investigate the chemical composition of exoplanet atmospheres using absorption spectra of light transmitted through those atmospheres.  We’ve succeeded in investigating the atmospheres of 288 planets, identifying 67 different chemical constituent gases.  The most distant planet where we have analyzed the atmospheric chemistry is at a distance of 124 light years.  That planet, K2-18b, has an atmosphere containing methane and traces of dimethyl sulfide.  On earth, dimethyl sulfide is produced by marine bacteria, and is a possible indicator of life. 

                                            Spectrometry of Exoplanet Atmospheres

                                                        James Webb Space Telescope

It’s worth reflecting for a moment about where we stand in our technological development.  It has been 122 years since the Wright brothers’ first flight, 119 years since the first radio broadcast, 68 years since the first satellite in space, 30 years since the discovery of the first exoplanet and only 9 years since the first chemical analysis of the atmosphere of an exoplanet.  Nine years.  Today we find it difficult to analyze the composition of planets orbiting close to their stars, but consider what we might be able to do in 300 or 500 years, presuming that human technology continues to advance.  

No new technology is needed to observe planets which are closer to their stars or more distant from Earth, at least until we reach a limit of interference from interstellar dust.  It just requires building bigger telescopes.  Within a radius of 100 light years, there are an estimated 10,000 to 60,000 stars.  The number of star systems increases exponentially with distance, so there are an estimated ten million stars within a radius of 1000 light years from Earth.   It’s easy to imagine that we could achieve an order of magnitude improvement in exoplanetary exploration in 100 years.  

Biosignatures
Certain molecules in the atmosphere are unstable combinations, and only occur together because one or both are being replenished through biological activity.  These combinations of coexisting molecules are considered likely indicators of extraterrestrial life.  Some of those combinations include methane and oxygen, methane and carbon dioxide (without carbon monoxide), nitrogen, methane and CO2 with liquid water, and high concentrations of oxygen and nitrogen.  A number of other individual compounds are independently considered biosignatures.  

Earth’s atmosphere has contained abundant coexisting oxygen and nitrogen for at least 2.3 billion years, in volumes that would be detectable by our own instrumentation, if our telescopes gave sufficient resolution.  Other biosignature molecules would probably also be evident to any extraterrestrial observer.  Compared to the 288 planets already analyzed for atmospheric chemistry, Earth’s atmosphere screams that life is robustly abundant on this planet.  And our atmosphere has been advertising that fact to the galaxy for the past 2.3 billion years.  

The Fermi Paradox, the Dark Forest Hypothesis and Atmospheric Chemistry
The Fermi Paradox is the apparent contradiction between our observation of billions of stars, representing billions of planets capable of hosting intelligent life, and the absence of any sign of extraterrestrial life or communication.  Reportedly, Enrico Fermi once abruptly asked his fellow physicists plaintively, “Where is everybody?”  I should note that others preceded Fermi in this question, notably Bernard de Bovier in 1686 (!!) and Jules Verne in 1865.

A variety of explanations have been proposed to explain the paradox.  These include (some of these are my own names):
>  The Great Filter – some evolutionary process greatly reduces the chance that intelligent life develops on other planets.  
>  The Prime Directive Hypothesis – the idea that advanced aliens have protected Earth from contact by extraterrestrial species.  This hypothesis is usually called The Zoo Hypothesis.
>  The Sagan Hypothesis – the idea that technological civilizations are very short-lived, tending to destroy themselves in military conflicts or environmental disasters.
>  The Bridge Too Far Hypothesis – the idea that interstellar distances and the limit of lightspeed are prohibitively great barriers to interstellar exploration.  
>  The City and The Stars Hypothesis – the idea that alien civilizations achieved immortality, and have no need or desire for exploration or expansion.  (Concept courtesy of my son and A.C. Clarke.)
>  The Gaseous Whale Hypothesis – the idea that most intelligent species evolve in oceans or gas giants, lacking the ability to use fire, observe other stars, to develop radio or space-faring technology due to environmental constraints.
>  The Smoke Signal vs. Telegraph Hypothesis – the idea that alien civilizations developed communication technologies far beyond the capability of humans to detect.
>  The Peacock’s Tail Hypothesis – the idea that human intelligence resulted from sexual selection rather than environmental adaptation, and that intelligence is as unique as a peacock’s tail.
>  The Childhood’s End Hypothesis – the idea that technological civilizations eventually progress beyond physical biological forms, and physical space is no longer of interest. 
>  The Dark Forest Hypothesis – the idea that extraterrestrial civilizations are abundant, but hiding from other potentially aggressive alien species.  
>  The Conehead, Lizard-People or X-Files Hypothesis – the idea that extraterrestrial beings exist and are living among us. 

The Dark Forest is the name of the second book in a science-fiction (or horror) trilogy by Liu Cixin,The Three-Body Problem.  The central concept of the trilogy (spoiler alert!) is that alien civilizations are abundant in the galaxy, but hiding out of fear of other aggressive, expansionistic species.  I think that the easy detectability of life through atmospheric chemistry should put this hypothesis to rest.  Technological alien civilizations have had over 2 billion years to recognize that Earth is life-bearing, and in an ideal environment for water/oxygen/carbon dioxide-based life.  If expansionistic space-faring aliens existed on any of the ten million stars within 1000 light years, they would have colonized the planet long ago.  Since we have no evidence that aliens ever visited the earth in the deep past, I think we have to conclude that the Fermi Paradox still holds.  

We don't know which of the proposed explanations for the Fermi Paradox is correct.  In my opinion, the Fermi Paradox results from the cumulative impact of several explanations.  I think that The Great Filter, The Sagan Hypothesis, The Bridge Too Far Hypothesis and the Peacock Tail Hypothesis together account for the absence of observable alien civilizations today.

Conclusion
There was exciting news from NASA’s Perseverance rover on Mars.  The robot explorer found evidence of two unusual minerals in a carbon-bearing mudstone deposited in a watery environment.  The rover’s spectrometry instrument detected “likely” vivianite, a hydrated iron phosphate mineral, and griegite, a magnetic iron sulfide mineral.  On earth, these minerals are only formed either in hydrothermal environments or by metabolic processes of microbes, including sulphate-reducing bacteria.  The host mudstones for the minerals show no evidence of hydrothermal heating.   Together with the previous discovery of long-chain alkanes (https://dougrobbins.blogspot.com/2025/03/curiosity-rover-on-mars-drills-hole.html), this evidence strongly supports the likelihood of ancient microbial life on Mars.  But ancient, sparse microbial life is a long way from technological, intelligent, alien species.

The Earth’s atmosphere is a signpost that the Earth is robustly teeming with life.  That signpost has been in place for at least 2.3 billion years for an alien civilization on any one of ten million stars within 1000 light years to observe.  And yet, there is no serious evidence that the planet has ever been visited by aliens.  There are no monoliths, no relict satellites, no abandoned bases on the moon and no tentacled aliens sipping Mai-Tais on the beaches of Maui.  All life on Earth appears to share a common evolutionary lineage, with no strains of bacteria possessing alternative DNA.  

We know from the history of our own planet that technological intelligence did not easily evolve.  About 3 billion years passed from the first appearance of life until the development of multi-cellular, animate life.  After animate life evolved, about 538 million years passed before an animal appeared which could use fire and make basic tools.   And after a species learned to make basic tools, use fire and exchange ideas through logical symbolic language, it was a million years before that species could write books, broadcast radio signals, build a heavier-than-air flying machine, or launch astronomical telescopes into orbit.  In retrospect, the odds for achieving each of those thresholds seems very low, given the time that passed before each step occurred.

 The Fermi Paradox is robustly upheld.  There are no aliens crouched around an experimental television, watching the first episode of “I Love Lucy” and planning an invasion.  Our galaxy is not the galaxy that is shown in Star Trek, teeming with competing intelligent alien species.  Millions of planets are within range of seeing that something interesting was happening on Earth for billions of years, but if anyone visited, they left no trace.  All of the evidence and reasonable deduction says that we are alone.  

References
The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats, Lammer et al, 2019.
https://pubmed.ncbi.nlm.nih.gov/31314591/

Redox-driven mineral and organic associations in Jezero Crater, Mars, Hurowitz et al, 2025.
https://www.nature.com/articles/s41586-025-09413-0

Charts of Atmospheric CO2, Carbon Isotopes, Oxygen and Methane

 I started making charts of atmospheric CO2 in 2009, when the global average CO2 concentration was 386 ppm.  I updated my charts in 2012, at 392 ppm, and in 2017, at 405 ppm, and at the end of 2021, at 418 ppm. 

The monitoring stations are located from the far north, at 82° N in Canada to the South Pole.  Scripps Institute has managed most of these stations since the 1950s, first under the direction of Charles Keeling, and later under his son, Ralph Keeling.  I also included records from a few obsolete legacy stations that were operated by foreign governments.  I standardized my chart displays using cool colors to represent the Northern Hemisphere, and warm colors for the Southern Hemisphere.

The amplitude of the CO2 seasonal cycle varies with latitude, from high amplitude in far northern latitudes to very little amplitude at the South Pole.  The seasonal cycle is driven by seasonal plant growth and decay on lands with temperate climate, which are concentrated in the Northern Hemisphere.  Agriculture, which is also concentrated in the Northern Hemisphere, also contributes to the seasonal cycle.  I took advantage of this for my standard display, overlaying low amplitude over higher amplitude traces, so that all traces can be seen.

In general, CO2 concentration in the atmosphere is growing exponentially, a fact noted by Isaac Asimov in 1959.  In 2009, I made an exponential function, beginning at the pre-industrial CO2 concentration of 280 ppm in 1800, with an eyeball-fit to the data from 1957 to 2009.  Here’s the function, and the chart beginning in 1800, updated with CO2 data through 2021.  This chart has the “hockey stick” impression that characterizes many climate-change charts.

CO₂ concentration, ppm = e^(n*0.001854) + 280, where n = the number of months since Jan. 1800

This function would predict that global CO2 would pass 450 ppm in January, 2032 (ten years from now), and pass 500 ppm in August, 2043.

The exponential function seems to be slightly overstating the rate of CO2 growth since 2009, so I tried an alternate formula for the forecast in coming decades, a second-degree polynomial with a least-squares fit to the global average CO2 from 1974 to 2009.  That formula is CO2 in ppm = 0.000104*x²+0.0897*x+331.66, where x is the number of months from July, 1974.  This formula predicts global CO2 will pass 450 ppm in June, 2034, and pass 500 ppm in July, 2050. 

Certainly, these forecasts are simple extrapolations, and include none of the analysis of policies and economics which should be the basis of forecasting.  But it’s worth noting that my exponential forecast from 13 years ago is pretty much right on the money, overshooting by only one or two parts per million.  The last thirteen years has seen unprecedented growth in renewable energy technologies, but so far without significant impact on the rate of CO2 growth.  Here are the two forecasts on the same chart.

The seasonal cycle can easily be filtered from the data, leaving the long-term trend at each station.  From this, it’s easy to see that the Northern Hemisphere leads the Southern Hemisphere in rising CO2.  About 90% of fossil fuel burning happens in the Northern Hemisphere, and CO2 accumulates in the far north, while dispersing to the south. 

The difference in concentration from the far north to the South Pole has been increasing as larger volumes of fossil fuels are burned each year, from about 3 ppm in the 1980s to over 5 ppm now.  The chart below shows the difference in the one-year time-averaged CO2 concentration measured in Alert, Canada, at latitude 82° North, and the South Pole. 

The amplitude of the seasonal cycle has also been increasing in the far north.  The amplitude of the cycle increased from 15 ppm to 20 ppm since the mid-1970s.  This probably reflects increased agriculture and farm productivity in the Northern Hemisphere as world population has doubled.  Previous work showed that seasonal fossil-fuel use is volumetrically inadequate to produce the change in the atmospheric CO2 seasonal cycle.  https://dougrobbins.blogspot.com/2012/04/modeling-global-co2-cycles.html

Carbon comes in two common naturally occurring isotopes, C¹² and C¹³.  Various processes, including life processes, sort the isotopes, favoring the accumulation of one or the other isotope.  Photosynthesis favors C¹², so everything with carbon derived from plants, including lumber, your mashed potatoes, you, me, and fossil fuels is enriched in C¹².  Scientists use a measure of the C¹³/C¹² ratio written as d¹³C , and called delC13.  As fossil fuels are burned the C¹²-enriched carbon in CO2 changes the ratio of these isotopes in the atmosphere, lowering the value of delC13.  DelC13 continued to fall from 2009 to 2021, reflecting a growing fraction of carbon from fossil fuels in the atmosphere. 

Carbon isotopes in the atmosphere are also affected by the seasonal cycle of plant growth on the temperate land mass of the Northern Hemisphere.  As plants grow during the northern summer, the lighter isotope C¹² is preferentially removed from the atmosphere, and returned during the winter months as plants decay.

After filtering the seasonal cycle, we see that the Northern Hemisphere leads the Southern Hemisphere in falling DelC13.  As an aside, the residual fluctuations in the trend have a strong correlation to the Oceanic Nino Index (ONI), reflecting sea surface temperatures in the Pacific.  https://dougrobbins.blogspot.com/2013/11/carbon-isotopes-in-atmosphere-part-ii.html

Interestingly, if all of the carbon released by fossil fuels stayed in the air, the DelC13 value would be much lower, about -13, instead of -8.5.  The measured dilution of carbon with the isotope signature of fossil fuels provides a way of estimating the volume of all carbon reservoirs exchanging carbon with the atmosphere.  Currently, the reservoirs freely exchanging carbon with the atmosphere have a carbon mass of about 5200 gigatonnes, before accounting for additional carbon in the system from new burning of fossil fuels.  That’s about 6 times the mass of carbon currently in the atmosphere.  https://dougrobbins.blogspot.com/2013/11/how-big-is-carbonsphere.html

Atmospheric oxygen is also influenced by burning of fossil fuels.  Oxygen is consumed, causing atmospheric O2 to fall.  The atmosphere is about 21% oxygen, and the decline is only about 0.08%, so there is no threat to breathing.  Still, the decline can be measured precisely.  The decline in oxygen is reported in units per meg, which is equivalent to ppm in this range of values.

After filtering the seasonal cycle, we see that the Northern Hemisphere leads the Southern Hemisphere in oxygen decline, because most fossil fuels are burned in the Northern Hemisphere.  The total volume of oxygen decline is very close to the expected consumption of oxygen considering the reported volumes of fossil fuels burned and deforestation, as reported in this previous post.  https://dougrobbins.blogspot.com/2019/12/understanding-source-of-rising.html

Atmospheric methane (C4) is also increasing as a result of human emissions.  Methane is a much more powerful greenhouse gas than CO2, but has a shorter lifespan.  CO2 has a half-life of 120 years, while methane has a half-life of about 10 years.  This is why the climate scientists use the parameter GWP (global warming potential) to represent the different strength of various greenhouse gases over an effective time frame.  The GWP of CO2 equals 1, by definition, for all time intervals.  For methane, the warming potential over 20 years (GWP-20) is 84 – 87, and over 100 years is 28 – 36.  Over shorter intervals, methane is an even stronger greenhouse gas.  Currently, methane concentration in the atmosphere is about 1.9 ppm (i.e. 1900 ppb).  In absolute terms, methane warmed the earth by about 0.52 W/m2, compared to 2.11 W/m2 for CO2, for the latest year reported by NOAA, 2020.  All other greenhouse gases combined contributed another 0.55 W/m2.  Methane also has a seasonal cycle in both hemispheres with high values in the summer and low values in the winter, but I don’t know the explanation for the seasonal cycle. 

As the concentrations of CO2 and methane in the air rise, the atmosphere will absorb heat at a faster rate, leading to destructive climate change.  Temperatures and climate change will not stabilize until carbon emissions reach zero.  I will update my charts on carbon emissions when summary data for 2021 is released in the BP Statistical Summary of World Energy in July.  Apart from a small pandemic-related decline in emissions in 2020, the world continues to add CO2 to the atmosphere at an ever-increasing rate.  If the world had acted to reduce emissions three decades ago, simply reducing emissions might have been a reasonable policy.  However, in our current situation, outright elimination of carbon emissions is required to avoid some level of catastrophic consequences. 

Globally, we need to reduce emissions to 50% by 2035, and to zero some time between 2050 and 2070.  I am very pessimistic that we have the public understanding or political will to reach these goals.  As Bill Gates wrote in 2021, "To avoid a climate disaster, we have to get to zero greenhouse gas emissions….The case for zero was, and is, rock solid.  Setting a goal to only reduce our emissions—but not eliminate them—won’t do it.  The only sensible goal is zero.”

References:

CO2, CO2 carbon isotopes, oxygen and methane data, including obsolete CO2 stations

https://scrippsco2.ucsd.edu/data/atmospheric_co2/sampling_stations.html

https://scrippso2.ucsd.edu/data.html

https://gml.noaa.gov/dv/data/

https://data.ess-dive.lbl.gov/view/doi:10.3334/CDIAC/ATG.015

https://www.osti.gov/dataexplorer/biblio/dataset/1409297

https://carbonmapper.org/data/

Isaac Asimov, "No More Ice Ages?" prediction and commentary on global warming,
in Fantasy and Science Fiction, Jan. 1959, republished in Fact & Fancy, 1962 and Asimov on Chemistry, 1974. 

Global Warming Potential

https://www.epa.gov/ghgemissions/understanding-global-warming-potentials#:~:text=Methane%20(CH4)%20is%20estimated,uses%20a%20different%20value.).

GWP-20 for methane = 84 to 87; GWP-100 for methane = 28 to 36 (also reported as 25)

Radiative Forcing for various greenhouse gases

https://gml.noaa.gov/aggi/aggi.html