The rate has accelerated
A widely cited conservative analysis found modern vertebrate extinction rates up to around 100 times the estimated background rate, even using assumptions designed to avoid exaggerating the crisis.
The global assessment is broader than extinctions alone
IPBES concluded that nature is declining globally at rates unprecedented in human history and that around one million animal and plant species face threatened extinction, many within decades, unless drivers are reduced.
Why terminology causes arguments
Past ‘Big Five’ mass extinctions are defined from the geological record by very large fractions of species disappearing. Today’s event has not yet reached the final magnitude of those completed events, so some researchers prefer ‘incipient sixth mass extinction’ while others say the process is already under way.
The trajectory is not fixed
Habitat protection, restoration, invasive-species control, sustainable harvest, pollution reduction and climate mitigation can still prevent many extinctions. Geological analogies should not be treated as destiny.
Research record
IPBES — Global Assessment Report on Biodiversity and Ecosystem ServicesIntergovernmental assessment · open sourceCeballos et al. (2015) — accelerated modern species lossesPeer-reviewed · open sourceAre We Living Through a Sixth Mass Extinction?
Direct answer: Yes in the sense that human-driven extinction rates and population losses are exceptionally elevated and consistent with an incipient mass-extinction process. No in the sense that the full losses of a completed Big Five event have not yet occurred.
The biodiversity crisis is empirically real; the semantic boundary around ‘sixth mass extinction’ should not obscure the measured acceleration of loss.