"Every indication is, this was done with the best of intentions but not the best of judgment," said spokesman Mike Townsley in a telephone interview from Mexico.
He said the organization was deeply concerned that the incident would offend its supporters, and apologized. Greenpeace does not accept contributions from companies or governments and is funded entirely by individuals.
Responsibility lies partly with the employee and partly with the organization, Townsley said.
The employee was a financial expert whose job included currency trades to protect the organization from fluctuations in the market. He was not qualified, however, to undertake trades on the scale he did. In addition, he ignored company rules, under which he should have checked with supervisors and probably hired an external expert for advice on how to manage risks.
"Hindsight is 20/20, but we believe if he had followed rules and procedures this wouldn't have happened," Townsley said.
Townsley said the employee was fired for ignoring rules, not for the loss itself, which although sizeable compared with donor contributions amounts to a fraction of the organization's annual revenues of around 300 million euros.
He said Greenpeace is taking steps to strengthen financial controls to prevent other such incidents.
The group said it would absorb the loss by cutting planned investments in "infrastructure" and pledged not to reduce spending on campaigns.