"Plastic is our biggest enemy," says Stephanie Pauwels. Everything around her looks like paradise: Deep blue sea, bright sandy beach and a wild mangrove landscape. The vibration of nature is literally heard and felt here. Pauwels is the director of "Corales del Rosario," a huge nature reserve on a gigantic archipelago off Cartagena, Colombia. Amidst this magic, she shows the German president that despite the pristine beauty, the idyll deceives. Nature is under threat from mass tourism, climate change, dynamite fishing — and mountains of plastic.

Johannes Vogel, director of the Museum of Natural History in Berlin, also warns of the devastating effects of plastic waste on the environment. It makes an impression. Steinmeier listens to the sobering reality and demands that governments around the world finally — and seriously — deal with the issue of plastic waste: "It is late, but all the more necessary."

A few hours later, Steinmeier visits the Pontifical Catholic University of Ecuador, where he is giving a speech on the occasion of the 250th birthday of German explorer and naturalist Alexander von Humboldt. The hall is filled with scientists, students and a host of political figures. Humboldt’s voyages across Latin America garnered him a reputation of pioneer and hero in much of the continent. Now the German president and and his wife, Elke Büdenbender, are embarking on a trip to trace his journey.

Read more: Berlin's Humboldt forum set to open

Andean expedition kicks off in Cartagena

Alexander von Humboldt was just 29 years old when he fulfilled his dream of undertaking a research trip through Latin America. His approach: To search for connections between things, to understand the world through close observation, to collect data and facts in order to feel, measure, and experience nature.

In 1799 he set sail from Spain, stopping along many outposts of its global empire; on March 30, 1801, he arrived in Cartagena, sailing past the same magnificent mangrove forests that are threatened today.

For Humboldt, the Colombian port city served merely as a stopover for another daredevil adventure: He wanted to explore the Andes and, with his companion, French explorer and botanist Aime Bonpland, travel to Lima. On foot. Almost 4,000 kilometers (2,485 miles).

Today it is almost impossible to imagine the drudgery such a journey entailed: Fierce snowstorms on the mountain ranges, tropical heat in the valleys. They made their way along insanely narrow paths, their mules packed with food, measuring instruments and records, as well as the animal and plant samples they had collected.

"They had to walk barefoot for long stretches; they had bloody feet. Their shoes and clothes were simply not made for that," says Humboldt biographer Andrea Wulf, who is accompanying the German president on his Latin America trip. "Unlike other explorers, Humboldt did not travel with a grand entourage and he did not let himself just be carried along."

Read more: Kehlmann's book 'Measuring the World' reveals Humboldt's exploring spirit

Changing the face of science

Already 200 years ago, Humboldt understood that humans influence the climate and cause soil erosion when they cut down the rainforest and practice monoculture. He is rightly regarded as the father of ecology, says Steinmeier.

At the same time, the explorer succeeded in inspiring his readers and listeners. He was a pioneer of science communication. "Humboldt emerged from the ivory tower and sober communications of a scientist, and brought the emotional world into science," says Hans-Christian Pape, President of the Alexander von Humboldt Foundation, which today allows outstanding scientists from all over the world to network and share knowledge.

Humboldt, a consummate renaissance man, was also involved in political matters. He strongly condemned slavery in the colonies; with the utmost respect he met indigenous people and stood up for their freedom. "He strongly criticized the Church because he saw how poorly the missionaries treated indigenous peoples," says biographer Wulf. "He marched through endless silver mines in Peru and Mexico and repeatedly denounced the gruesome treatment and exploitation of miners." Back in Europe, Humboldt ranted against colonialism so vehemently that the British refused him travel to India. "Why should they let such a difficult Prussian into their colony?"says Wulf with a smile.

Read more: Scientists race to name unknown species before biodiversity disappears

Humboldt's outstanding achievement, however, was his ability to grasp the big picture from the smallest detail he examined. "At that time, scientists crept into their increasingly narrow disciplines, and Humboldt did just the opposite — he tried to bring everything together," says his biographer. "Humboldt revolutionized our understanding of nature, describing nature as a living organism."

In early January 1802, Humboldt stopped in Quito — an ideal base for researchers interested in volcanic activity. Here he climbed every volcano he could reach in his quest to understand how the earth was created. Fast forward to 2019, and during the German president's trip to Quito it's no longer about the creation of the earth, but the threat of its destruction. "The entire ecosystem of the earth is threatened," warns Steinmeier. "We only have this one planet, so we cannot go on like this." Environmental protection is now the responsibility of every human being: This is Alexander von Humboldt's message for the 21st century, Steinmeier underlines.