The facts and the practical responses to them are separate issues, but some Catholics do not understand that or object to it – and there is no lack of evidence for that in Bishop Marcelo Sanchez Sorondo's inbox.

The bishop is chancellor of both the Pontifical Academy of Sciences and the Pontifical Academy of Social Sciences. The two academies are holding a workshop Feb. 27-March 1 on preserving biodiversity.

In January, the bishop began receiving messages objecting to the invitation the academies extended to Paul R. Ehrlich, president of the Center for Conservation Biology at Stanford University. The letters of protest highlight Ehrlich 's controversial book, "The Population Bomb," published in 1968, and his advocacy of strong population-control measures, including through abortion.

Ehrlich, a biologist, is not a member of either pontifical academy but has been invited to speak at the workshop because of his studies in the field of conservation biology.

"Naturally, someone can say, 'Oh, look who they have invited to the Vatican,' but the positive side is that he can help us find the truth in the theme we are discussing," Bishop Sanchez told Catholic News Service. Ehrlich is one of two people asked to speak about how "consumption preferences, population numbers, technology (and) ecosystem productivity" impact biodiversity.

The Vatican has long acknowledged the fact of global population growth, has shared concern about increased poverty rates in the fastest-growing regions of the world and accepts the scientific evidence that the growing population has had a negative effect on the environment.

However, in evaluating policies to respond to the scientific fact of population growth and environmental destruction, the Vatican insists on recognition of the sacredness of every human life, respect for human dignity and trust in the human capacity to change and to innovate. Where some scientists would favour population-control policies, modern popes consistently have argued that the problem is less about the number of people living on the planet and more about human selfishness, the unfair distribution of resources and a lack of will to find creative solutions.

Before making moral evaluations of policy, the Pope and bishops need to know the scientific facts. The Vatican gets those from scholars with scientific expertise, regardless of their religious beliefs or their opinions on the policy implications of the scientific facts.

The object of the upcoming workshop and Ehrlich's speech is not population control, Bishop Sanchez said. It is how to respond to the call of Pope Francis in "Laudato Si'" to protect the diversity of plants and animals God created.

Objecting to the invitation of a scientist recognized as an expert in his field "is not logical," the bishop said. "Critics are only following the logic of attack. And it's always the same people."

Combined, the two pontifical academies have about 100 members – including two dozen Nobel laureates – but Bishop Sanchez said he does not know how many of them are Catholic. Academicians are nominated by the Pope after vetting by the Vatican Secretariat of State.

Bishop Sanchez said, "I am interested only in their scientific achievements. Many of them are Nobel laureates. We look for excellence, for scientists from a variety of disciplines and, third, for those with a global reputation."

"Their private opinions are their opinions," he said. "What counts is the conclusions that we will draw," but to imagine that the conclusions will contradict church teaching on the gift and sacredness of human life "is crazy."

Losing patience with the complaints, Bishop Sanchez said it is not just that the critics seem to be afraid of scientific findings and the church's ability to respond to them reasonably, "they are afraid of their own shadows."

"Truly, I just don't understand them. Through dialogue we are able to obtain much more than they are with their policy of always criticizing others," he said.