We might not notice a 'mass' extinction

Although current biodiversity loss rates may already align with the criteria for a mass extinction, the authors argue that the long timescales required for such events to fully materialize and the lag in species discovery mean humanity is at high risk of failing to recognize the crisis as it unfolds, potentially absolving current society of responsibility for catastrophic future consequences.

The Gist

The Big Picture: The "Invisible" Disaster

Imagine the Earth's biodiversity (all the plants, animals, and bugs) as a massive, bustling library containing millions of books. Some of these books are famous bestsellers (like lions or oak trees), but most are obscure, dusty manuscripts that no one has ever read or even cataloged yet.

The authors of this paper are asking a terrifying question: What if the library is burning down right now, but because so many of the books are still in the basement, we won't realize the fire is a "mass extinction" until it's too late?

The Core Problem: The "Unseen" Books

We know many species are dying out. We have a list of about 1,000 species that are officially confirmed extinct. But scientists estimate there could be anywhere from 1.8 million to over 100 million species on Earth.

Here is the catch: We haven't even found most of them yet.

• The Analogy: Imagine you are trying to count how many people are leaving a stadium. You have a list of 1,000 people who left, but you don't know how many people are inside the stadium. If there are only 2,000 people total, losing 1,000 is a disaster. But if there are 100 million people inside, losing 1,000 seems tiny.
• The Reality: Because we haven't "cataloged" (named and described) most of the species, they can go extinct without us ever knowing they existed. They vanish into the dark before we even know their names.

The Race: The Librarian vs. The Fire

The paper runs a simulation of a race between two forces:

1. The Fire (Extinction): Species are dying off at a rapid rate due to climate change, habitat loss, and pollution.
2. The Librarian (Taxonomists): Scientists are working hard to find, name, and write down the details of new species.

The Result of the Race:
In nearly 50% of the scenarios the authors tested, the fire wins. The species go extinct before the Librarian can write their names in the catalog.

If a species goes extinct before it is ever described, it leaves zero evidence. It's like a book burning in a locked room where no one ever saw the cover. When we look back in 1,000 years, we won't see a "mass extinction" in the records because the records were never made.

The "75% Rule" and the Time Trap

To officially declare a "Mass Extinction" (like the one that killed the dinosaurs), scientists usually look for a specific threshold: 75% of all life must disappear.

The paper points out a scary time mismatch:

• The Timeline: Based on current rates, it might take thousands or even tens of thousands of years for us to lose 75% of life.
• The Problem: Politicians and governments usually only plan for 5 or 10 years. They might say, "Look, we haven't lost 75% yet, so we are fine."
• The Trap: By the time we finally cross that 75% line and say, "Oh no, it's a mass extinction," the damage will be irreversible. We will have missed the window to stop it.

The "Frog in Boiling Water" Effect

The authors use a classic metaphor: The frog in the pot.
If you throw a frog into boiling water, it jumps out immediately. But if you put it in cold water and slowly turn up the heat, the frog doesn't notice the temperature rising. It stays in the pot until it's too late to escape.

• The Heat: The slow, steady loss of species.
• The Frog: Humanity.
• The Danger: We are so focused on short-term problems (like the economy or next year's election) that we don't notice the "water" is getting hotter. We might not even realize we are in a mass extinction until the water is already boiling.

Why This Matters (Even if We Don't See It)

The paper argues that just because we can't prove a mass extinction is happening right now (because we lack the data), doesn't mean it isn't happening.

• The "Holocaust" Analogy: The authors compare this to a historical tragedy that was never recorded. If a genocide happened but no one wrote it down, future historians might say, "It never happened." But the reality is, the people still died.
• The Consequence: If we assume "no evidence means no problem," we will stop trying to save nature. We will let the library burn down, and future generations will inherit a world that is empty, quiet, and broken, unaware of how rich and diverse it used to be.

The Bottom Line

We are likely already on the path to a sixth mass extinction. The scary part isn't just that species are dying; it's that we might be too blind to see it happening.

Because we haven't named most of the species, they can disappear without a trace. By the time we have enough data to officially say, "Yes, this is a mass extinction," it will be thousands of years too late to fix it. We need to act now, not because we have perfect proof, but because the risk of missing the warning signs is too high to ignore.

Technical Summary

1. Problem Statement

The scientific community largely agrees that global biodiversity is declining rapidly, with extinction rates significantly exceeding background levels. However, there is a paradox: while the sense of emergency is high, the number of documented extinctions remains low (~1,000 species), leading some to question whether a "sixth mass extinction" is actually occurring.

The core problem addressed by the authors is the epistemological gap between current extinction rates and the ability to empirically confirm a mass extinction event.

• Definition Challenge: A mass extinction is typically defined as the loss of $\ge$ 75% of global diversity within a "short" geological timeframe (< 2 million years).
• The Detection Paradox: To confirm a mass extinction, one must document that 75% of species have gone extinct. However, if the majority of species on Earth are currently undescribed (unknown to science), they can go extinct without ever being recorded.
• The Question: Will humanity be able to collect sufficient data to confirm a mass extinction is underway, or will the event occur "under the radar" due to the race between species loss and taxonomic discovery?

2. Methodology

The authors employed a stochastic thought experiment combining three dynamic models to simulate the "race" between taxonomic description and biodiversity loss from the year 2026 to 10,000 (and up to 100,000 in some analyses).

A. Global Diversity Estimates

They utilized a wide range of estimates for total global animal diversity ($S$) to account for uncertainty, ranging from 1.8 million (near current known counts) to 163.2 million (high-end estimates including undescribed invertebrates).

B. Extinction Rate Modeling

Using IUCN Red List data, they derived three extinction rate scenarios ($\mu$) representing the proportion of species lost per year:

1. Optimistic: Only confirmed Extinct (EX) and Extinct in the Wild (EW).
2. Intermediate: EX, EW, and Critically Endangered (CR) species flagged as "Possibly Extinct."
3. Pessimistic: All EX, EW, and CR species (assuming an extinction debt).

• Rates ranged from 0.00004 to 0.00053 species per evaluated species per year.
• They used a birth-death model ($N_t = N_0 e^{(\lambda - \mu)t}$) where speciation ($\lambda$) was fixed at $10^{-7}$ year$^{-1}$.

C. Species Description Trajectory

They modeled the rate of new species descriptions based on historical trends (1970–2020), which showed a linear increase of ~77 new species per year, reaching ~13,110 descriptions in 2026.

• Saturation Effect: They introduced a functional response term to account for the decreasing ease of finding new species as the pool of undescribed species shrinks.
• Equation: $D(t) = [a + b(t - 1970)] \times (1 - C_t/S)^\alpha$
  • Where $C_t$ is cumulative descriptions, $S$ is total diversity, and $\alpha$ (set to 0.5) modulates the slowing of discovery.

D. Simulation Design

• Factorial Grid: 100 unique combinations of 10 diversity estimates $\times$ 10 extinction rates.
• State Variables: The model tracked annual changes in:
  • Total extant species ($div$).
  • Cumulative described species ($des$).
  • Undocumented extinctions ($ue$): Species lost before being described.
  • Documented extinctions ($doc\_ext$).
• Thresholds:
  • Mass Extinction Realized: $div < 0.25S$ (75% loss).
  • Confirmation Possible: $des > 0.75S$ (75% of total diversity described).
  • Confirmation Impossible: $ue > 0.25S$ (25% of total diversity lost without ever being described).

3. Key Contributions

1. Quantification of the "Invisible Extinction" Risk: The study provides the first quantitative assessment of the probability that a mass extinction will occur without ever being empirically confirmed due to the "knowledge gap" of undescribed species.
2. Temporal Mismatch Analysis: It highlights the disconnect between the geological timescale required for a mass extinction to unfold (thousands of years) and the short-term horizons of policy and legislation.
3. Refinement of the "Sixth Extinction" Paradigm: The authors argue that while current extinction rates are consistent with mass extinction trajectories, the confirmation of such an event is statistically unlikely under many realistic scenarios.

4. Key Results

A. The Probability of Missing the Event

• 49% of Scenarios: In nearly half of the simulated parameter combinations (specifically those with higher estimates of global diversity and higher extinction rates), undocumented extinctions exceeded 25% before 75% of species could be described.
• Consequence: In these scenarios, the condition to confirm a mass extinction (documenting 75% of diversity) becomes mathematically impossible because the "denominator" (total species) has been reduced by unknown losses.

B. Timing of Confirmation vs. Collapse

• Time to Mass Extinction: Under current rates, a 75% loss of diversity is projected to occur between 2,604 and 34,808 years from now.
• Time to Confirmation: In the scenarios where confirmation is possible (low diversity estimates), the 75% description threshold is reached around year 2,930.
• The Gap: In high-diversity scenarios, the threshold for "impossible to confirm" (25% undocumented loss) is crossed much earlier (median year ~3,143) than the threshold for "mass extinction realized."

C. Escalation Risk

Using fossil data (marine genera over 500 million years), the authors modeled the probability of a biodiversity crisis escalating to a mass extinction.

• The probability of escalation rises sharply once ~30% of species are lost.
• Current policies accepting large risks of biosphere collapse are compared to irrational fears of rare events (e.g., shark attacks), arguing that the risk of a mass extinction is unacceptably high by engineering and safety standards.

5. Significance and Implications

• Policy Paralysis: The finding that we might not notice a mass extinction could be misused to justify inaction ("If we can't prove it's happening, why act?"). The authors argue the opposite: the inability to document the event does not negate the reality of the crisis.
• Irreversibility: The process of biodiversity loss is non-linear (exponential decay). Early extinctions are more critical to prevent because they trigger co-extinctions and network collapses that accelerate the process.
• Ethical Responsibility: Even if the full "mass extinction" threshold is not reached for thousands of years, the trajectory is set. Future generations will inherit a world they perceive as "normal" but which is ecologically impoverished, unaware of the potential for a better state.
• Redefining Urgency: The paper suggests that waiting for empirical proof of a mass extinction is a flawed strategy. The focus must shift to the rate of loss and the prevention of local ecosystem collapse, which has immediate impacts on human well-being, regardless of whether the global 75% threshold is ever formally documented.

Conclusion: The paper concludes that there is a roughly 50/50 chance that humanity will fail to document a mass extinction as it unfolds. This statistical uncertainty should not absolve society of responsibility but rather serve as a warning that the crisis is already underway, even if the full historical record remains incomplete.